[codex] Restore parse model serialization

[pragnyanramtha]

Summary

• restore model_to_dict support for raw response objects that expose parse()
• guard parse recursion by tracking object identity and falling back to __dict__ when parsing fails or returns itself
• unskip and extend serialization tests for parsed models, self-referential parse results, and parse failures

Root cause

safe_serialize already routed objects with parse() into model_to_dict, and the helper docstring promised raw API response support, but the parse branch was commented out after recursion problems. Those objects fell through to __dict__, which could lose the parsed response payload expected by instrumentation code.

Validation

• uv run pytest tests/unit/test_serialization.py -q
• uv run ruff check agentops/helpers/serialization.py tests/unit/test_serialization.py

Note: uv run --locked ... currently fails because the checked-in uv.lock wants to be rewritten by this uv version; this PR intentionally leaves the lockfile untouched.

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