test: stabilize nested plugin timeout

[Puppo]

Summary

• allow enough startup time for nested plugins when the test suite runs concurrently
• keep the assertion focused on the deliberately unresolved innermost plugin
• preserve the upstream 100% coverage gate without production-code exclusions

The previous 10 ms budget could expire the parent plugin before nested registration completed under concurrent load, producing a failure that named first instead of third.

Validation

• npm test — 331 tests passed, including TypeScript validation
• npm run test:coverage — 100% statements, branches, functions, and lines
• npm run lint
• isolated timeout test passed 25 consecutive runs