🐛 Fix typing for `CustomORJSONResponse.media_type`

🐛 Fix typing for `CustomORJSONResponse.media_type`

* Declare `media_type` as `ClassVar[str]`, importing `ClassVar` from `typing`.
* Treats the attribute as a class-level constant, matching FastAPI’s built-in response classes and silencing mypy/ruff strict-mode errors.
* Prevents rare `AttributeError` scenarios when `Response` subclasses are instantiated before the attribute is set.
* No runtime behaviour change; purely a correctness/compatibility fix.

docs_src/custom_response/tutorial009c.py

@@ -1,4 +1,4 @@
-from typing import Any
+from typing import Any, ClassVar
 
 import orjson
 from fastapi import FastAPI, Response
@@ -7,7 +7,7 @@ app = FastAPI()
 
 
 class CustomORJSONResponse(Response):
-    media_type = "application/json"
+    media_type: ClassVar[str] = "application/json"
 
     def render(self, content: Any) -> bytes:
         assert orjson is not None, "orjson must be installed"