Post-release: bump rel/4.3 to 4.3.1 / 2.3.2 and mark APIs shipped

[Amaury Levé (Evangelink)]

Post-release housekeeping for the rel/4.3 branch following the MSTest 4.3.0 + Microsoft.Testing.Platform 2.3.1 release.

Version bump (eng/Versions.props)

• VersionPrefix: 4.3.0 → 4.3.1
• TestingPlatformVersionPrefix: 2.3.1 → 2.3.2

The darc-updated MSTestVersion / MicrosoftTestingPlatformVersion lines are intentionally untouched.

Mark APIs & analyzers as shipped (reflecting the 4.3.0 release)

• Ran eng/mark-shipped.ps1: every src/**/PublicAPI.Unshipped.txt entry was moved into the sibling PublicAPI.Shipped.txt, and each Unshipped file was reset to a single #nullable enable line.
• Analyzer release notes: moved the ### New Rules / ### Changed Rules tables from AnalyzerReleases.Unshipped.md into AnalyzerReleases.Shipped.md under a new ## Release 4.3.0 section at the top (matching how main records it). AnalyzerReleases.Unshipped.md now contains only its two ; header lines.

Verified via git diff that only eng/Versions.props, the PublicAPI.Shipped.txt / PublicAPI.Unshipped.txt files, and the two AnalyzerReleases.*.md files changed.

[github-actions]

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✅ 22/22 dimensions clean — no findings.

Summary: This is a standard post-release housekeeping PR. All changes are purely mechanical:

• Version bump (eng/Versions.props): 4.3.0 → 4.3.1 / 2.3.1 → 2.3.2 — correct next-patch increment for the development cycle.
• PublicAPI tracking (14 file pairs): Every entry removed from PublicAPI.Unshipped.txt appears in the corresponding PublicAPI.Shipped.txt. All Unshipped files are correctly reset to a single #nullable enable line.
• Analyzer release tracking: Rules MSTEST0064–0068, MSTEST0070, MSTEST0071 (new) and MSTEST0050 (changed severity) correctly moved to AnalyzerReleases.Shipped.md under ## Release 4.3.0.

No source code, test, or behavioral changes — dimensions 1–3, 5–16, 18–19, and 22 are N/A. Dimensions 4 (Public API), 17 (Documentation), 20 (Build Infrastructure), and 21 (Scope) verified clean.