Run the native UIA provider tests in windows-shell unit validation

[coneilen]

Summary

Tools/windows/Tests/AccessibilityProvider.Native.Tests.ps1, the native C++ UIA provider regression suite, was orphaned: no task in validate.ps1 and no workflow invoked it. So native accessibility provider changes (such as lane D2's workspace-restart-session mapping for #774) could not get exact-head CI evidence. Lane D2 found this on 2026-10-14.

This PR runs the suite once per windows-shell unit run, on shard 0, so it executes in exact-head CI.

Changes

• Tools/windows/validate.ps1: after the sharded WindowsShell.Tests.ps1 contract in the unit part, shard 0 runs AccessibilityProvider.Native.Tests.ps1 through Invoke-Native, so a non-zero exit fails the task. The suite isn't sharded, so it runs exactly once across the 3-shard CI matrix and in local unsharded runs.

Test plan

RED: Select-String origin/main Tools/windows/validate.ps1 for AccessibilityProvider.Native.Tests.ps1 -> 0 invocations; the native suite never ran in CI
GREEN: Select-String edited validate.ps1 -> 1 invocation (shard 0); local native build of the suite on main via MSVC cl -> Native accessibility provider: 176 checks, 0 failures
REGRESSION: pwsh Tools/windows/Tests/ValidationRunner.Tests.ps1 -> ValidationRunner.Tests.ps1: PASS (384 multi-project contracts, 50 positive, 334 negative); git diff --check -> clean; bash Tools/ci/tests/classify-changes.test.sh -> 59 passed, 0 failed

Local environment caveat.

• On this machine, vcvars64.bat exits 1 because the optional ext\ConnectionManagerExe.bat can't find the VC Linux component, so the script's call vcvars && cl chain stops locally.
• The 176/0 result above came from the same cl command chained with &.
• The hosted windows-2022 runner is the authoritative check: the exact-head shard-0 job must show the Native accessibility provider: N checks, 0 failures line with N greater than 0.
• macOS make test and make check were not run, because no Swift was touched.

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