chore: Return explict None type for DELETE endpoints

The HTTP response code will become 204 No Content instead of 200.

PiperOrigin-RevId: 796123628

Author: DeanChensj

src/google/adk/cli/adk_web_server.py

@@ -457,6 +457,14 @@ async def create_session(
       logger.info("New session created")
       return session
 
+    @app.delete("/apps/{app_name}/users/{user_id}/sessions/{session_id}")
+    async def delete_session(
+        app_name: str, user_id: str, session_id: str
+    ) -> None:
+      await self.session_service.delete_session(
+          app_name=app_name, user_id=user_id, session_id=session_id
+      )
+
     @app.post(
         "/apps/{app_name}/eval_sets/{eval_set_id}",
         response_model_exclude_none=True,
@@ -606,7 +614,9 @@ async def update_eval(
         "/apps/{app_name}/eval_sets/{eval_set_id}/evals/{eval_case_id}",
         tags=[TAG_EVALUATION],
     )
-    async def delete_eval(app_name: str, eval_set_id: str, eval_case_id: str):
+    async def delete_eval(
+        app_name: str, eval_set_id: str, eval_case_id: str
+    ) -> None:
       try:
         self.eval_sets_manager.delete_eval_case(
             app_name, eval_set_id, eval_case_id
@@ -733,12 +743,6 @@ async def list_eval_metrics(app_name: str) -> list[MetricInfo]:
             status_code=400, detail=MISSING_EVAL_DEPENDENCIES_MESSAGE
         ) from e
 
-    @app.delete("/apps/{app_name}/users/{user_id}/sessions/{session_id}")
-    async def delete_session(app_name: str, user_id: str, session_id: str):
-      await self.session_service.delete_session(
-          app_name=app_name, user_id=user_id, session_id=session_id
-      )
-
     @app.get(
         "/apps/{app_name}/users/{user_id}/sessions/{session_id}/artifacts/{artifact_name}",
         response_model_exclude_none=True,
@@ -813,7 +817,7 @@ async def list_artifact_versions(
     )
     async def delete_artifact(
         app_name: str, user_id: str, session_id: str, artifact_name: str
-    ):
+    ) -> None:
       await self.artifact_service.delete_artifact(
           app_name=app_name,
           user_id=user_id,