chore: Throw 409 already exist error when trying to create a session with existing id

Resolves https://github.com/google/adk-python/issues/2950

PiperOrigin-RevId: 821362008

src/google/adk/cli/adk_web_server.py

@@ -741,7 +741,7 @@ class AdkWebServer:
           is not None
       ):
         raise HTTPException(
-            status_code=400, detail=f"Session already exists: {session_id}"
+            status_code=409, detail=f"Session already exists: {session_id}"
         )
       session = await self.session_service.create_session(
           app_name=app_name, user_id=user_id, state=state, session_id=session_id
@@ -763,6 +763,13 @@ class AdkWebServer:
             app_name=app_name, user_id=user_id
         )
 
+      if req.session_id and await self.session_service.get_session(
+          app_name=app_name, user_id=user_id, session_id=req.session_id
+      ):
+        raise HTTPException(
+            status_code=409, detail=f"Session already exists: {req.session_id}"
+        )
+
       session = await self.session_service.create_session(
           app_name=app_name,
           user_id=user_id,