test(windows): stabilize subscription lock contention

[wgqqqqq]

Summary

Stabilize the cross-process subscription-auth file-lock regression test on loaded Windows runners.

The child process now confirms an actual OS-level lock contention before notifying the parent, replacing the previous fixed 100 ms scheduling assumption. Test-only timeout headroom covers Windows process startup, antivirus scanning, and metadata sync_all latency without changing production lock behavior.

Type and Areas

Type: regression fix / test

Areas: Rust, AI adapters, subscription authentication, Windows CI

Motivation / Impact

The Windows Rust matrix intermittently failed separate_process_waits_for_metadata_commit_before_reconciliation because the test announced child startup before the child had attempted the lock and used overlapping five-second timeouts.

No direct user-facing change. Production file-lock behavior is unchanged.

Verification

• pnpm run fmt:rs
• git diff --check
• Target test repeated 20 times locally: 20/20 passed
• cargo test --locked -p bitfun-ai-adapters --features subscription-auth --lib subscription_auth: 63 passed
• Fork Windows CI Run subscription authentication tests: passed
• Fork Rust Build Check (windows-latest): passed
• Fork Rust / CLI Validation: passed
• Evidence: https://github.com/wgqqqqq/BitFun/actions/runs/33232411871

The fork run had an unrelated Web UI assertion failure in ConfigPageLayout.test.tsx; all Rust and CLI matrix jobs and the Rust/CLI summary gate passed.

Reviewer Notes

This only changes code under #[cfg(test)]. No persisted formats, product behavior, or remote-scenario behavior are affected.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable (not applicable; test-only change).