[codex] Fix data parser cache leaks

[ObservedObserver]

Summary

• Replace method-level @lru_cache() on data parser cached properties with per-instance cached_property.
• Cover pandas parser raw_fields and field_metas with a GC regression test.
• Verify parser instances remain picklable after cached properties are loaded.

Root Cause

functools.lru_cache stores calls in the decorated function wrapper. For instance methods and properties, that cache key includes self, so parser instances are retained by the class-level cache after callers drop their references. Those parser instances keep their DataFrames alive, causing memory growth in long-running processes that parse many datasets.

Notes

This reimplements the core fix from #724 against current main, avoiding the merge conflicts and avoiding a per-instance threading.Lock so parser pickling keeps working.

Fixes #723

Validation

• ./venv/bin/python -m pytest tests/test_data_parsers.py -q
• ./venv/bin/python -m pytest -q
• ./venv/bin/python -m ruff check .

[ObservedObserver]

Closing in favor of #736, which preserves the original #724 author commits while keeping the same final fix and passing CI.