[codex] Skip parsing incomplete response text

[fengjikui]

Summary

• avoid structured-output parsing for response message text unless the message is completed
• preserve incomplete output text while leaving parsed as None
• add a regression test for incomplete JSON output in parse_response

Fixes #2486.

Root cause

parse_response attempted to parse every message.output_text item with the requested structured output type. For incomplete responses, that text can contain partial JSON, so Pydantic raises a validation error instead of returning the incomplete response payload.

Validation

• uv run --with pytest --with pytest-asyncio --with respx --with inline-snapshot pytest tests/lib/responses/test_responses.py::test_parse_incomplete_output_text -q -o addopts=""
• env -u HTTP_PROXY -u HTTPS_PROXY -u ALL_PROXY -u http_proxy -u https_proxy -u all_proxy uv run --with pytest --with pytest-asyncio --with respx --with inline-snapshot pytest tests/lib/responses/test_responses.py -q -o addopts=""
• uv run --with ruff ruff format --check src/openai/lib/_parsing/_responses.py tests/lib/responses/test_responses.py
• uv run --with ruff ruff check src/openai/lib/_parsing/_responses.py tests/lib/responses/test_responses.py
• uv run --with pyright --with pytest --with pytest-asyncio --with respx --with inline-snapshot pyright src/openai/lib/_parsing/_responses.py tests/lib/responses/test_responses.py