Replace `dict` by `Mapping` on `HTTPException.headers`

[rijenkii]

Summary

See also: https://github.com/fastapi/fastapi/discussions/12786

HTTP Headers cannot be represented by a dict, as they can have duplicate entries. Example: Set-Cookie.

As such type of starlette.exceptions.HTTPException's headers field should be typing.Mapping instead of dict, just like starlette.responses.Response's headers field: https://github.com/encode/starlette/blob/c2e3a39b09a613553ee03586589ed9cd0fbf07f3/starlette/responses.py#L27-L38

Checklist

• [x] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [x] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [x] I've updated the documentation accordingly.