fix: retry transient file locks in release test cleanup

[HelloThisWorld]

Summary

• Release run 30107145962 (first v1.1.2 attempt) failed only in cleanup: test-portable.ps1 hit Access to the path '...\Microsoft.Terminal.Control.dll' is denied while removing its temp extract root, after every functional gate had passed. A just-terminated winTerm process (or an antivirus scan of freshly written binaries) can keep a mapped file locked for a short moment.
• Adds Remove-DirectoryWithRetry (up to 5 attempts with 250/500/750/1000 ms backoff, early exit when the directory is already gone, rethrow on final failure) to scripts/winterm/test-portable.ps1 and scripts/winterm/test-installer.ps1, and routes their recursive temp-directory cleanup through it.
• Only the two scripts that launch processes from their temp directories are changed. verify-release-layout.ps1 never launches the payload and has never flaked, so it is left as is. No product code, schemas, or workflows are touched.

Validation

• Behavior checks against the real function (extracted via AST from each script): permanently locked directory still throws after the attempt budget; unlocked directory is removed immediately; a lock released after ~700 ms is survived without throwing — the exact release-run scenario. All six checks passed.
• scripts/winterm/test.ps1 -Suite Smoke -Configuration Release -Platform x64 — passed.
• git diff --check — clean.
• Compiled suites and the real installer/portable lifecycle run on the PR workflows (winterm-validation.yml, winterm-full-build.yml); no local native toolchain is available.

Notes

• The temporary-root safety guard (refusing to delete outside %TEMP%) is unchanged and still runs before any removal.
• A permanent lock still fails the job loudly; only transient locks are absorbed.