Make the native UIA test harness robust to cleanup locks and vcvars extension failures

[coneilen]

Summary

#802 made the windows-shell unit part run AccessibilityProvider.Native.Tests.ps1. That passes on hosted runners, but on developer machines it turned passing runs into failures, for two harness reasons:

1. Cleanup lock. Push-Location $scratch made the scratch folder the pwsh process's working directory at the native call. Pop-Location doesn't change the process directory back, so the final Remove-Item failed with "being used by another process", and the script exited 1 after its checks had passed. Lane D2 hit this on a full unit run (1201/1201 tests and 55/55 sections passed, yet overall exit 1); I reproduced it locally.
2. vcvars exit code. vcvars64.bat exits 1 when an optional extension (ext\ConnectionManagerExe.bat, the VC Linux component) is absent, even though the MSVC environment is usable. call vcvars && cl therefore never compiled on such machines.

Changes

Tools/windows/Tests/AccessibilityProvider.Native.Tests.ps1:

• change directory inside cmd (cd /d), with no Push-Location, so the pwsh process never holds the scratch folder;
• after vcvars, require a resolvable cl (where cl) instead of trusting vcvars's exit code. A real compile or test failure still fails the run;
• clean up with a bounded retry (10 attempts, 500 ms apart), then warn, so a transient antivirus lock can't turn a passing run into a failure.

Test plan

RED: pwsh Tools/windows/Tests/AccessibilityProvider.Native.Tests.ps1 on main 13758a7 (local) -> exit 1, Remove-Item could not remove the scratch folder because it is being used by another process; cmd /c call vcvars64.bat -> exit 1 (ConnectionManagerExe init FAILED)
GREEN: pwsh Tools/windows/Tests/AccessibilityProvider.Native.Tests.ps1 on this branch (local) -> exit 0, Native accessibility provider: 176 checks, 0 failures, no scratch folder left; negative: -VcVars pointing to a no-op batch file with MSVC removed from PATH -> exit 1 (Native accessibility provider tests failed: 1), no scratch left
REGRESSION: pwsh Tools/windows/Tests/ValidationRunner.Tests.ps1 -> ValidationRunner.Tests.ps1: PASS; git diff --check -> clean; bash Tools/ci/tests/classify-changes.test.sh -> 59 passed, 0 failed

Exact-head CI shard 0 must still show Native accessibility provider: N checks, 0 failures with N greater than 0. macOS make test and make check were not run, because no Swift changed.

Checklist

• I have read the Contributing Guidelines
• I have signed off my commits (git commit -s) per the DCO
• Tests pass locally (make test)
• Code follows the existing style (make check)
• I added the test/contract before the implementation and observed the intended RED failure