fix: coerce APIStatusError.code to str to match Optional[str] annotation

[Varshith-Kali]

Summary

Fixes #3531 — APIStatusError.code is annotated Optional[str] but construct_type(type_=Optional[str], value=...) passes through non-str values unchanged (int, bool, etc.), and the cast(Any, ...) silences the type checker. Downstream code that trusts the annotation and calls exc.code.strip() crashes with AttributeError.

Fix

Replace the construct_type + cast pattern with an explicit str() coercion guarded by a None check:

• Non-None code values (int, bool, float, etc.) → str(value)
• None → None (unchanged)
• String codes pass through str() as identity (unchanged)

Testing

Verified all six coercion cases pass:

• {"code": 404} → "404"
• {"code": "rate_limit"} → "rate_limit"
• {"message": "nope"} → None
• {"code": true} → "True"
• {"code": null} → None
• Not a dict body → None

[amirhosseinghanipour]

Same fix as the one in #3532. Two notes:

1. Line 1 has an unintended change. Worth reverting that. It's noise unrelated to the fix and will likely trip up diffing/linting.
2. No test was added for this. fix: normalize API error codes to strings #3532 includes one. Consider deferring to that PR since it covers the same fix with a regression test.

[Varshith-Kali]

Thanks @amirhosseinghanipour for the review. You're right — #3532 already covers this with a proper regression test, and this PR has an unintended line-1 diff. Closing in favor of #3532.