test(ffe): frame mocked CDN responses for fetch clients

tests/test_the_test/test_mock_ffe_agentless_backend.py

@@ -34,6 +34,7 @@ def test_mock_ffe_agentless_backend_serves_fixture_and_tracks_metadata(worker_id
             timeout=5,
         )
         response.raise_for_status()
+        assert response.headers["Content-Length"] == str(len(response.content))
 
         payload = response.json()
         assert payload["data"]["type"] == UFC_RESPONSE_TYPE

utils/docker_fixtures/_mock_ffe_agentless_backend.py

@@ -194,6 +194,7 @@ def _handle_config(self) -> None:
                 self.send_response(status_code)
                 for key, value in headers.items():
                     self.send_header(key, value)
+                self.send_header("Content-Length", str(len(body)))
                 self.end_headers()
                 if body:
                     self.wfile.write(body)
@@ -229,10 +230,12 @@ def _handle_responses_control(self) -> None:
 
     def _write_json(self, status_code: HTTPStatus, payload: dict[str, Any] | MockFFEAgentlessBackendStatus) -> None:
         with contextlib.suppress(BrokenPipeError, ConnectionResetError):
+            body = json.dumps(payload).encode("utf-8")
             self.send_response(status_code)
             self.send_header("Content-Type", "application/json")
+            self.send_header("Content-Length", str(len(body)))
             self.end_headers()
-            self.wfile.write(json.dumps(payload).encode("utf-8"))
+            self.wfile.write(body)
 
 
 def _has_auth(headers: Mapping[str, str]) -> bool: