fix: coerce API error code/param/type to str

[Manny7717]

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Changes being requested

Fixes #3531 — APIStatusError.code is annotated Optional[str] but can be an int at runtime.

Problem: APIError.__init__ builds code/param/type with the lenient deserializer:

self.code = cast(Any, construct_type(type_=Optional[str], value=body.get("code")))

construct_type neither coerces nor rejects values that don't match the target type — it returns them unchanged — and the cast(Any, ...) suppresses the type checker. So when a server sends an integer error code (observed from api.openai.com under load, and routinely from OpenAI-compatible gateways), exc.code is an int while the annotation promises str | None. Downstream code that trusts the annotation (e.g. exc.code.strip()) crashes with AttributeError after the API failure already happened.

Fix: coerce non-None values to str in the exception constructor, so the runtime value always matches the documented annotation. The raw value remains available on APIError.body for consumers that need it. Applied consistently to code, param and type (all three share the same pattern).

Tests: added test_api_status_error_code_is_coerced_to_str to both the sync and async client suites (end-to-end via a mocked 400 response with "code": 404). Verified revert-proven: both new tests fail on the pre-fix code and pass with the fix. Full tests/test_client.py: 200 passed, 2 skipped. ruff check . and mypy src/openai/_exceptions.py clean.

Note: src/openai/_exceptions.py is handwritten — it carries no Castiron "File generated from our OpenAPI spec" marker — so this change won't be overwritten by the generator.

Additional context & links

• Issue: APIStatusError.code is typed Optional[str] but can be an int at runtime #3531
• Drafted with Hermes Agent (deepseek-v4-flash); every line reviewed and verified by Manny7717 (tests run, revert-proven, lint + type-checked).