Backport client statefulness capability fix to rel/4.4

[Amaury Levé (Evangelink)]

Summary

Backports #11037 to rel/4.4.

This is part 1 of 4 and should be merged first.

Validation

• .\\build.cmd -pack -bl — passed with 0 warnings and 0 errors.
• Client capability, JSON serialization, adapter run-settings, and VSTest bridge tests — 184 passed.
• Server-mode acceptance coverage passed as part of the targeted backport validation.

All MSBuild-based validation commands produced binary logs.

[github-actions]

🧵 Parallel-safety audit — PR #11293

Parallelization — one row per test assembly audited:

Test assembly	Scope	Workers	Analyzer coverage
MSTestAdapter.UnitTests	off (bespoke TestContainer engine — no MSTest parallel scheduler)	n/a	n/a
Microsoft.Testing.Extensions.VSTestBridge.UnitTests	MethodLevel	CPU count	coverable once the parallel-safety analyzers ship (assembly attribute)
Microsoft.Testing.Platform.ServerMode.Client.Sources.UnitTests	MethodLevel	CPU count	coverable once the parallel-safety analyzers ship (assembly attribute)
Microsoft.Testing.Platform.UnitTests	MethodLevel	CPU count	coverable once the parallel-safety analyzers ship (assembly attribute)
Microsoft.Testing.Platform.Acceptance.IntegrationTests	MethodLevel	CPU count	coverable once the parallel-safety analyzers ship (assembly attribute)

No .runsettings / testconfig.json opt-ins were found for any of these assemblies, and this PR does not touch any parallelization-state file (no .runsettings, testconfig.json, Directory.Build.*, or [assembly: Parallelize]/[DoNotParallelize] change) — the config-files list for this PR is empty.

Findings: A (global-state) 0 · B (paths) 0 · C (declaration) 0 · D (over-serialization) 0 — by severity: Critical 0 · High 0 · Warning 0 · Info 0.

This backport (#11293, backporting #11037 to rel/4.4) changes 7 test files, all narrowly scoped to the IsStateful/DeclaredIsStateful nullable-capability rename and its new "undeclared" test cases:

• MSTestRunSettingsTests.cs (MSTestAdapter.UnitTests, TestContainer-based — no live parallel risk regardless).
• RunSettingsPatcherTests.cs, ObjectModelConvertersTests.cs (Microsoft.Testing.Extensions.VSTestBridge.UnitTests).
• MtpServerClientTests.cs (Microsoft.Testing.Platform.ServerMode.Client.Sources.UnitTests) — the new tests construct a fresh FakeMtpServer/TcpListener bound to an ephemeral port (new TcpListener(IPAddress.Loopback, 0)) per test instance, so there is no shared port or shared state across tests.
• JsonTests.cs, ClientCapabilitiesExtensionsTests.cs (Microsoft.Testing.Platform.UnitTests) — pure serialization/deserialization assertions against locally-constructed records; no shared statics.
• ClientCapabilities.cs (record definition used by acceptance tests) — a type-only change (bool → bool?), no test logic touched.

I checked each changed test method and lifecycle member for: process-global state mutation (env vars, CWD, console, culture, static fields — category A), shared filesystem paths (category B), and [ResourceLock]/[DoNotParallelize] declaration mismatches (category C). None of the changed tests mutate environment variables, current directory, console state, culture, or any shared static; none touch the filesystem; and none of the surrounding classes declare [ResourceLock]/[DoNotParallelize] that this PR's changes would invalidate. All new/modified assertions operate on locally-scoped mocks, dictionaries, and freshly-constructed objects.

Nothing to flag for parallel-safety in this PR.

_{Advisory only — heuristic, non-blocking. Re-run with /parallel-audit. This audit answers "is it parallel-safe?"; for testability, smells, or flakiness see the detect-static-dependencies / test-smell-detection / test-anti-patterns analyses.}

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[github-actions]

🧪 Expert test review — PR #11293

This PR renames ClientCapabilitiesService.IsStateful to DeclaredIsStateful (nullable, three-state: declared true / declared false / undeclared) and adds a GetIsStateful() extension so undeclared-vs-explicit-false can be distinguished. The changed/added tests exercise this new tri-state distinction across the MSTest adapter's MSTestRunSettings, the VSTestBridge RunSettingsPatcher, the wire-level JSON (System.Text.Json and Jsonite) serialization/deserialization paths, the MtpServerClient wire payload, and the new ClientCapabilitiesExtensions.GetIsStateful() helper itself.

Grade	Test	Mutation	Notes	How to improve
A (90–100)	new ClientCapabilitiesExtensionsTests. GetIsStateful_ ForwardsCustomImplementation	2/2 killed	Data-driven test proves the extension falls back to IsStateful for non-ClientCapabilitiesService implementers.	—
A (90–100)	new ClientCapabilitiesExtensionsTests. IsStateful_ UndeclaredCapability_ DefaultsToFalse	1/1 killed	Confirms the DeclaredIsStateful: null case defaults IsStateful to false via the record.	—
A (90–100)	new MSTestRunSettingsTests. UndeclaredNonVisualStudioClient DoesNotSetDesignMode	1/1 killed	Verifies undeclared statefulness on a non-VS client falls back to false (no VS heuristic).	—
A (90–100)	new MSTestRunSettingsTests. UndeclaredVisualStudioClient SetsDesignModeForBackwardCompatibility	1/1 killed	Verifies the legacy-VS backward-compat fallback only fires when statefulness is undeclared.	—
A (90–100)	new MSTestRunSettingsTests. StatelessVisualStudioClient DoesNotSetDesignMode	1/1 killed	Verifies an explicit false declaration overrides the VS backward-compat fallback.	—
A (90–100)	new RunSettingsPatcherTests. Patch_UndeclaredNonVisualStudioClient_ DoesNotSetDesignMode	1/1 killed	Mirrors the MSTest-adapter case for RunSettingsPatcher.Patch directly.	—
A (90–100)	new RunSettingsPatcherTests. Patch_UndeclaredVisualStudioClient_ SetsDesignModeForBackwardCompatibility	1/1 killed	Covers the VS legacy-compat branch when statefulness is undeclared.	—
A (90–100)	new RunSettingsPatcherTests. Patch_StatelessVisualStudioClient_ DoesNotSetDesignMode	1/1 killed	Covers the explicit-false-overrides-VS-heuristic branch for RunSettingsPatcher.Patch.	—
A (90–100)	new MtpServerClientTests. InitializeAsync_DefaultOptions_ LeaveStatefulnessUndeclared	1/1 killed	End-to-end over a real loopback connection; asserts the wire request omits isStateful by default.	—
A (90–100)	new MtpServerClientTests. SerializeClientCapabilities_ UndeclaredStatefulness_OmitsProperty	1/1 killed	Verifies null IsStateful serializes by omitting the wire property entirely.	—
A (90–100)	new MtpServerClientTests. SerializeClientCapabilities_ DeclaredStatefulness_IncludesProperty	2/2 killed	Data-driven true/false cases confirm the declared value round-trips onto the wire payload.	—
A (90–100)	mod JsonTests. Deserialize_InitializeRequest_ WithIsStatefulFalse_StjPath_ SurfacesStatelessClient	1/1 killed	Renamed/repurposed to cover explicit false; matches the StJ deserializer's JsonValueKind.False branch.	—
A (90–100)	new JsonTests. Deserialize_InitializeRequest_ WithoutIsStateful_StjPath_ LeavesCapabilityUndeclared	1/1 killed	Confirms a missing isStateful property deserializes to null, not a default.	—
A (90–100)	new JsonTests. Deserialize_InitializeRequest_ WithInvalidIsStateful_StjPath_Throws	1/1 killed	Non-boolean JSON value hits the deserializer's MessageFormatException default arm.	—
A (90–100)	new JsonTests. Deserialize_InitializeRequest_ WithNullIsStateful_StjPath_Throws	1/1 killed	JSON null for isStateful is neither True/False, so it throws as designed.	—
A (90–100)	mod JsonTests. Deserialize_ClientCapabilities_ WithIsStatefulFalse_JsonitePath_ SurfacesStatelessClient	1/1 killed	Jsonite-path counterpart of the StJ false-case rename; matches SerializerUtilities's bool-cast branch.	—
A (90–100)	new JsonTests. Deserialize_ClientCapabilities_ WithoutIsStateful_JsonitePath_ LeavesCapabilityUndeclared	1/1 killed	Confirms missing key on the Jsonite path deserializes to null.	—
A (90–100)	new JsonTests. Deserialize_ClientCapabilities_ WithInvalidIsStateful_JsonitePath_Throws	1/1 killed	Non-bool value hits the Jsonite deserializer's type-check throw branch.	—
A (90–100)	new JsonTests. Deserialize_ClientCapabilities_ WithNullIsStateful_JsonitePath_Throws	1/1 killed	JSON null value is not a bool, so the Jsonite path throws as designed.	—

No actionable inline findings were raised — every reviewed test asserts a value that traces to a distinct, verifiable branch of the DeclaredIsStateful/GetIsStateful() behavior (declared-true, declared-false, undeclared, and invalid-input paths), uses the assertion style native to its project (AwesomeAssertions in the adapter suite, MSTest Assert elsewhere), and each pseudo-mutation on the relevant production branch is killed.

_{This advisory comment was generated automatically. Grades are heuristic
and informational — they do not block merging. Suggestions on the Files
changed tab can be applied with one click. Re-run with
/review-tests.}

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