Add independent log parser fixtures

[bingmokaka]

Summary

Adds independent hand-written parser fixtures for the legacy log aggregator so JSON, plain text, nginx, and malformed log paths are validated without parser-generated samples.

Changes

• Added fixture files under tests/fixtures/log_aggregator/ for JSON, plain text, nginx access logs, and a malformed/unsupported line.
• Added tests/test_log_aggregator_fixtures.py using unittest to verify timestamp, level, service/format, and key parsed fields.
• Fixed two parser issues exposed by the fixtures:
  • Route nginx parsing before the plain-text fallback in LogAggregator.
  • Read nginx remote_user from the correct captured field.

Testing

• python -m unittest tests.test_log_aggregator_fixtures -v passes: 5 tests.
• python tools/log_aggregator.py --input tests/fixtures/log_aggregator/nginx.log --output diagnostic/log_aggregator_smoke.json --format json passes locally and reports one nginx error entry.
• python build.py was attempted but the Windows environment lacks the full multi-language toolchain and npm, so the all-module build crashes when spawning npm install for frontend.
• python build.py -m backend was also attempted; it reached diagnostic finalization but failed because the bundled encryptly diagnostic pack step returned an error in this Windows environment. No new diagnostic artifact was committed.

Checklist

• Relevant modules affected by these changes build locally
• Tests pass locally
• Diagnostic build log is committed in this PR
• Documentation has been updated, if applicable
• Configuration or schema changes are documented, if applicable
• No generated build artifacts are committed, except the required diagnostic build log
• Changes are scoped to the PR purpose and avoid unrelated cleanup
• Security, privacy, and error-handling implications have been considered

---

• I would like to request that my diagnostic build log is removed before merging

Closes #5