Preserve explicit -> None return annotations in @depends-wrapped endpoint signatures

[Copilot]

@depends was rebuilding wrapped function signatures with a type-hint-resolved return annotation, which converted explicit -> None into NoneType. In FastAPI, this could cause incorrect response model inference and trigger 204 No Content assertion errors unless users added manual workarounds (response_model=None or removing return typing).

• Signature reconstruction fix

  • Updated depends() to keep the original function’s return annotation verbatim when creating __signature__.
  • This preserves None exactly as authored and avoids changing FastAPI route semantics for no-content endpoints.

• Regression coverage

  • Added a focused test for an endpoint declared as async def ... -> None with status_code=204 and a @depends decorator.
  • The test verifies the route is valid and returns an empty 204 response without requiring workaround configuration.

# before (could become NoneType through hint resolution)
new_signature = original_signature.replace(
    parameters=tuple(all_params.values()),
    return_annotation=hints.get("return", original_signature.return_annotation),
)

# after (preserve authored return annotation)
new_signature = original_signature.replace(
    parameters=tuple(all_params.values()),
    return_annotation=original_signature.return_annotation,
)

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[FieryRMS]

I'll get put out of my job cuz of copilot 🫡
but PR LGTM

ps: i dont have a job