test: stabilize logFileContent on Windows CI

[s2005]

Summary

• replace file-exists polling with content-readiness polling in tests/unit/logFileContent.test.ts
• wait for expected header/output markers before asserting
• increase wait timeout to better tolerate slower Windows filesystem timing

Why

storeLog() writes files asynchronously, so checking only file existence can read partial content on Windows runners and produce intermittent failures.

Validation

• npm test -- --runInBand tests/unit/logFileContent.test.ts
• 80 repeated runs of the same test file all passed locally