Improve typing for add_exception_handler

[Ragug]

Summary

This improves type inference for add_exception_handler by allowing handlers
to accept specific Exception subclasses instead of only Exception.

For example, the following now type-checks correctly:

class MyException(Exception):
    ...

def handler(request: Request, exc: MyException) -> Response:
    ...

app.add_exception_handler(MyException, handler)

This is achieved using overloads while preserving the existing runtime behavior.

No runtime changes are introduced.

Note: This PR intentionally does not attempt to improve typing for the
exception_handlers mapping, as its key-dependent typing cannot be fully
expressed in Python's type system.

Related discussion: #2403

Checklist

• I understand that this PR may be closed in case there was no previous discussion.
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[Ragug]

Hi @Kludex, I’d appreciate it if you could review this and let me know if any changes are needed. Looking forward to your feedback.