fix(exceptions): coerce APIError.code to str to match Optional[str] annotation

[okxint]

Summary

APIError.code is typed Optional[str] but could silently hold an int at runtime when the API returns a numeric error code.

Root cause:

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

construct_type returns the raw value unchanged when the runtime type doesn't match the target type. Wrapping with cast(Any, ...) suppresses the type checker, so a numeric code (e.g. 1001) escapes into .code as an int. Any downstream code that treats .code as a string (.strip(), .startswith(), string comparison) will raise AttributeError: 'int' object has no attribute 'strip'.

Fix:

raw_code = body.get("code")
self.code = str(raw_code) if raw_code is not None else None

Explicit str() coercion guarantees the annotation is always honoured regardless of what the API sends.

Checklist

• Fix matches Optional[str] annotation on APIError.code
• None is preserved when code key is absent or null
• No behaviour change for APIs that already send string codes
• Removes the cast(Any, ...) type-suppression workaround

[sylvesterkaczmarek]

This is the same runtime fix already open in #3611, but this version drops the regression that proves numeric codes are normalised. I'd either close this in favour of #3611 or carry over a focused int/string/None test. Otherwise the exact construct_type mismatch that motivated the change isn't pinned by CI.

[okxint]

Thanks for the review @sylvesterkaczmarek. Added a focused regression in the latest commit that pins int→str, str→str, and None→None coercion under CI. Happy to close in favor of #3611 if the maintainers prefer — but since #3611 still shows a formatting failure I figured it's worth keeping this one moving.