test: add independent log parser fixtures for JSON, text, and nginx formats (fixes #5)

[Fuhua-GIF]

Summary

Adds independent parser fixture tests for tools/log_aggregator.py, as requested in #5.

Changes

• Added tests/test_log_parser_fixtures.py: 5 tests with hand-written (NOT parser-generated) fixtures:
  1. JSON parser correctly parses 4 hand-written JSON log fixtures (various levels, fields, nested data)
  2. Text parser correctly parses 4 hand-written text log fixtures (INFO, ERROR, WARN, DEBUG)
  3. Nginx parser correctly parses 3 hand-written nginx access log fixtures (200, 500, 404 status codes)
  4. 5 malformed/unsupported lines (invalid JSON, random text, empty, truncated nginx, partial match) do not crash any parser
  5. Parser independence verified - each parser produces expected output fields for its format

Fixture Sources

All fixtures are hand-written based on real-world log formats:

• JSON fixtures follow structured logging conventions (timestamp, level, service, message)
• Text fixtures follow common syslog-style format (timestamp level [service] message)
• Nginx fixtures follow the standard nginx access log format (combined log format)
• Malformed fixtures represent common edge cases (invalid JSON, empty lines, truncated logs)

Testing

python3 tests/test_log_parser_fixtures.py

All 5 tests pass. No production parsing behavior was changed.

Checklist

• Independent fixtures for JSON, plain text, and nginx-style log lines
• Tests verify parser output for timestamp, level, service/format, and key fields
• At least one malformed/unsupported line case included (5 total)
• Existing log aggregation CLI behavior remains compatible
• PR description explains fixture source/intent and validation performed

Fixes #5