fix(wif): prevent credential refresh after cleanup

[abhinavkr26104]

Summary

• prevent an in-flight WIF refresh from recreating credential material after action cleanup
• add a regression test for a refresh that resolves after stop()

Audit finding

base-action/src/workload-identity.ts starts asynchronous OIDC refreshes with setInterval. Cleanup previously called clearInterval() and removed RUNNER_TEMP/claude-workload-identity, but did not cancel a refresh already awaiting core.getIDToken().

Reproduction:

1. Configure workload identity federation.
2. Let a scheduled refresh enter fetchIdentityToken().
3. Finish the action while that request is still pending.
4. The stop() cleanup removes the credential directory.
5. The pending refresh resolves and writes a new identity token and cache directory.

Expected: after stop(), no identity token or exchanged-credential cache is recreated.

Actual before this change: the asynchronous refresh could recreate credential material after cleanup, leaving secret-bearing files in the runner's per-job temporary directory.

The fix marks the handle stopped before cleanup and checks that state after the awaited token request, before calling setSecret() or writing files.

This is distinct from #1406/#1407, which addressed repeated OIDC exchanges across spawned Claude processes. I found no existing issue or PR covering this post-stop refresh race.

Fixes #1621

Validation

• bun test test/workload-identity.test.ts -t "does not recreate credentials when a refresh finishes after stop"
• bun test test/parse-sdk-options.test.ts
• bun run typecheck (root)
• bun run typecheck (base-action)
• bunx prettier --check base-action/src/workload-identity.ts base-action/test/workload-identity.test.ts
• git diff --check

Full repository test runs were also attempted; the checkout has several pre-existing platform/environment failures on Windows (CRLF-sensitive metadata tests, POSIX symlink/permission assertions, network image fixtures, and base README parsing), documented in the audit handoff.

[abhinavkr26104]

Updated the branch with current main in ad18f67. The WIF cleanup-race regression still passes, as do the root and base-action TypeScript checks, Prettier on the changed files, and git diff --check. The only failures in the full WIF test file locally are the two pre-existing Windows permission-mode assertions; they are unrelated to this change.