Update CES test to use stable model (#17037) (#1281)

[upstream:c0aaa132b342c244c2620fa8b3a2695f09a26f4f]

Signed-off-by: Modular Magician <magic-modules@google.com>

Author: modular-magician

ces_agent_basic/main.tf

@@ -71,7 +71,7 @@ resource "google_ces_agent" "ces_child_agent" {
   instruction = "You are a helpful assistant for this example."
 
   model_settings {
-    model       = "gemini-2.5-flash-001"
+    model       = "gemini-2.5-flash"
     temperature = 0.5
   }
 
@@ -116,7 +116,7 @@ resource "google_ces_agent" "ces_agent_basic" {
   instruction = "You are a helpful assistant for this example."
 
   model_settings {
-    model       = "gemini-2.5-flash-001"
+    model       = "gemini-2.5-flash"
     temperature = 0.5
   }