Replace dict by Mapping on HTTPException.headers

[rijenkii]

Summary

See also: fastapi/fastapi#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:

starlette/starlette/responses.py

Lines 27 to 38 in c2e3a39

	class Response:
	media_type = None
	charset = "utf-8"
	def __init__(
	self,
	content: typing.Any = None,
	status_code: int = 200,
	headers: typing.Mapping[str, str] | None = None,
	media_type: str | None = None,
	background: BackgroundTask | None = None,
	) -> None:

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• 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.