Fix CI: add missing v0.3.24 MCPG pin and Windows test compatibility

[dsyme]

Splits out the generic CI fixes from #37863 so they can land independently of that PR's behavior change.

What's fixed

test job — TestGetContainerPin_DefaultMCPImagesArePinned

Adds the ghcr.io/github/gh-aw-mcpg:v0.3.24 entry (with verified digest from GHCR) to pkg/actionpins/data/action_pins.json. The constant DefaultMCPGatewayVersion was bumped to v0.3.24 in #37708 but the embedded pin entry was missed, so the default-image-is-pinned test has been failing on main since.

Windows Integration: compile job (CWI workflow)

• compilation_with_nonexistent_file: relax the expected substring from no such file (POSIX-only) to failed to read file, which is what the engine itself emits on all platforms.
• absolute_path_not_allowed: pick an OS-appropriate absolute path so filepath.IsAbs returns true on Windows (C:\\absolute\\path).
• TestCompileWithIncludeWithEmptyFrontmatterUnderPty: skip on Windows where creack/pty returns 'unsupported'.

Windows Integration: version job (CWI workflow)

• TestVersionIsSetDuringBuild: build the throwaway test binary inside t.TempDir() with the correct .exe suffix on Windows, instead of hardcoding /tmp/gh-aw-test-version which doesn't exist as an executable path on Windows runners.

Verification

go test ./pkg/actionpins -run TestGetContainerPin # PASS
go test -tags=integration -run 'TestCompileWorkflow$|TestCompileWorkflows_WorkflowDirValidation|TestCompileWithIncludeWithEmptyFrontmatterUnderPty' ./pkg/cli/... # PASS
go test -tags=integration -run TestVersionIsSetDuringBuild ./cmd/gh-aw/... # PASS

These same fixes are also present in #37863 and will continue to land there if this PR merges first.

[pelikhan]

Landon is bringing #37885

[Copilot]

Pull request overview

Fixes CI failures by completing the embedded MCP gateway (MCPG) container pin set and making several integration tests OS-aware so they behave consistently on Windows runners.

Changes:

• Add the missing ghcr.io/github/gh-aw-mcpg:v0.3.24 container pin (digest + pinned reference) to the embedded action pins dataset.
• Adjust Windows integration tests for platform-specific behavior (PTY unsupported, absolute path detection, error substring differences).
• Build and execute the version-test binary using t.TempDir() and a .exe suffix on Windows.

Show a summary per file

File	Description
pkg/cli/compile_integration_test.go	Skip PTY-based compile integration test on Windows where PTY isn’t supported.
pkg/cli/compile_command_test.go	Use an OS-appropriate absolute path to keep filepath.IsAbs-based validation test portable.
pkg/cli/commands_compile_workflow_test.go	Expect a platform-neutral error substring for missing workflow files.
pkg/actionpins/data/action_pins.json	Add the missing MCPG v0.3.24 container pin entry (tag → digest).
cmd/gh-aw/version_test.go	Build the test binary into t.TempDir() and use .exe on Windows.

Copilot's findings

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• Files reviewed: 4/4 changed files
• Comments generated: 0

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[github-actions]

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