fix: coerce numeric API error codes to strings

[nightcityblade]

• I understand that this repository is auto-generated and my pull request may not be merged

Changes being requested

Normalize non-null API error codes to strings when constructing APIError, so the runtime value matches the documented str | None contract even when an endpoint or compatible gateway returns a numeric JSON code.

Add a focused regression test covering a numeric 404 error code.

Fixes #3531

Additional context & links

Validation:

• .venv/bin/python -m pytest tests/test_exceptions.py -q
• .venv/bin/ruff check src/openai/_exceptions.py tests/test_exceptions.py
• .venv/bin/ruff format --check src/openai/_exceptions.py tests/test_exceptions.py
• .venv/bin/mypy src/openai/_exceptions.py

[jbeckwith-oai]

Reviewed the error-construction boundary and regression coverage. Normalizing every non-null wire value to str keeps the runtime behavior aligned with the existing str | None public contract while preserving None, and the focused test covers the observed numeric-code case. The full repository lint/typecheck/import gate plus focused tests and formatting checks are clean.