Fix more mypy and gemini detected errors

Author: aneeshgarg

src/a2a/client/jsonrpc_client.py

@@ -702,7 +702,7 @@ async def get_task(
             http_kwargs=self.get_http_args(context),
             context=context,
         )
-        return response.result
+        return response.root.result
 
     async def cancel_task(
         self,
@@ -727,7 +727,7 @@ async def cancel_task(
             http_kwargs=self.get_http_args(context),
             context=context,
         )
-        return response.result
+        return response.root.result
 
     async def set_task_callback(
         self,
@@ -752,7 +752,7 @@ async def set_task_callback(
             http_kwargs=self.get_http_args(context),
             context=context,
         )
-        return response.result
+        return response.root.result
 
     async def get_task_callback(
         self,
@@ -777,7 +777,7 @@ async def get_task_callback(
             http_kwargs=self.get_http_args(context),
             context=context,
         )
-        return response.result
+        return response.root.result
 
     async def resubscribe(
         self,

src/a2a/server/apps/rest/rest_app.py

@@ -86,8 +86,8 @@ def _generate_error_response(self, error: A2AError) -> JSONResponse:
             f'{", Data=" + str(error.root.data) if error.root.data else ""}',
         )
         return JSONResponse(
-            f"{{'message': '{error.root.message}'}}",
-            status_code=404,
+            f'{{"message": "{error.root.message}"}}',
+            status_code=500,
         )
 
     def _handle_error(self, error: Exception) -> JSONResponse: