Add independent log parser fixtures and validation (+ fix 2 parser bugs)

[zhaojiazheng9-art]

Summary

Add hand-written log parser fixtures for JSON, text, and nginx formats to eliminate the 40% false-pass rate caused by parser-generated test data. Includes a dedicated validation script and fixes two parser bugs exposed by the new fixtures.

Changes

• tools/tests/fixtures/: New directory with hand-written fixture files
  • json_logs.fixture — 7 JSON log entries with standard and alternative key formats
  • text_logs.fixture — 6 plain text log entries with timestamps and [bracket] services
  • nginx_logs.fixture — 6 Nginx combined-format access log entries
• tools/tests/validate_parsers.py: New validation script that:
  • Tests timestamp, level, service, and key field extraction per format
  • Includes 8 malformed/unsupported line cases that must not crash parsing
  • Validates JSON key aliases (time→timestamp, severity→level, logger→service, etc.)
• tools/log_aggregator.py: Bug fixes
  • extract_service(): regex [\w+] → [\w-]+ to handle hyphenated service names
  • NginxLogParser: remote_user now reads group 3 instead of group 2 (was reading ident field)
• build.py: Fixed SyntaxError from backslash in f-string expression
• diagnostic/: Updated build diagnostic artifacts

Testing

$ python3 tools/tests/validate_parsers.py
==================================================
  Log Parser Validation
==================================================
  JSON parser (7 lines)     — 7 passed
  Text parser (6 lines)     — 6 passed
  Nginx parser (6 lines)    — 6 passed
  Malformed lines (8 cases) — 0 crashes
==================================================
  Results: 19 passed, 0 failed, 0 crashes
==================================================