Add BaseResponse class and ContentT generic to Response

[Kludex]

Continuation of #2547.

Extract BaseResponse from Response to separate responses that use render() (Response, HTMLResponse, JSONResponse, etc.) from those that don't (StreamingResponse, FileResponse). Currently StreamingResponse and FileResponse inherit from Response but never call render() — they only use shared infrastructure (headers, cookies, status code).

Response is now Response(BaseResponse, Generic[ContentT]), and JSONResponse is JSONResponse(Response[ContentT]).

StreamingResponse and FileResponse inherit from BaseResponse directly, and call super().__init__() instead of manually setting attributes.

Breaking changes:

• StreamingResponse and FileResponse no longer inherit from Response.
• Type aliases that used Response now use BaseResponse (e.g. HTTPExceptionHandler, RequestResponseEndpoint).