Fix release stages ignoring native command failures

[AGiorgetti]

Change

Release validation and early NuGet push failures could be hidden when later native commands reset LASTEXITCODE. Check each native exit immediately and report its operation or package before continuing.

Covers GitVersion, Windows toolchain/tests, both solution formats, every pack/API check/push, consumer restore/run, both trimmed execution branches, and AOT publish. The native test fixture is included in both solutions so clean builds use the requested configuration. Preserves #19's publishing gates, production approval, artifact dependencies, and the local helper's existing guards. Changelog entry is under Unreleased/Fixes.

Fixes #20.

Verification

• Test-first: 36 executable regressions produced 26 failures and 10 passes against the original workflow, zero skips; all eight success controls passed.
• Extracted PowerShell scripts execute native fixtures returning exit 23 at each affected position. Tests reject any later native call or promotion, including with pre-existing artifacts. No real NuGet endpoint is used.
• Corrected solution rebuild after cleaning fixture output: zero warnings/errors; expanded focused/repository/documentation checks: 220 passed, zero failed/skipped.
• Final full local suite: 844 passed, zero failed, one skip for the unavailable local Native AOT linker.
• Exact-head ordinary PR CI run 37326069684 passed at 5026e7a: Linux and Windows each passed 845 tests with zero failures/skips, including Native AOT. All six Roslyn matrix jobs passed. Linux sample smoke checks and five benchmark smoke cases passed.
• Pack & Validate, Verify Release Artifacts, and Publish NuGet Packages were all skipped in that PR run.

Ready for human review; retained as draft. No real NuGet push, release, tag push, workflow dispatch, or merge was performed.