fix(registry): update Gemini 3.1 Flash Lite model ID and add test for validation

Closes: #4391

internal/registry/model_definitions_test.go

@@ -16,6 +16,25 @@ func TestModelOverrideHeadersFromEmbeddedModels(t *testing.T) {
 	}
 }
 
+func TestGeminiVertexModelsUseFlashLiteReleaseID(t *testing.T) {
+	const releaseID = "gemini-3.1-flash-lite"
+	const previewID = releaseID + "-preview"
+
+	for _, model := range GetGeminiVertexModels() {
+		if model == nil {
+			continue
+		}
+		if model.ID == previewID {
+			t.Fatalf("Vertex model ID = %q, want release ID %q", model.ID, releaseID)
+		}
+		if model.ID == releaseID {
+			return
+		}
+	}
+
+	t.Fatalf("Vertex models do not contain %q", releaseID)
+}
+
 func TestWithXAIBuiltinsIncludesVideoPreviewModel(t *testing.T) {
 	models := WithXAIBuiltins(nil)
 

internal/registry/models/models.json

@@ -735,13 +735,13 @@
       }
     },
     {
-      "id": "gemini-3.1-flash-lite-preview",
+      "id": "gemini-3.1-flash-lite",
       "object": "model",
       "created": 1776288000,
       "owned_by": "google",
       "type": "gemini",
-      "display_name": "Gemini 3.1 Flash Lite Preview",
-      "name": "models/gemini-3.1-flash-lite-preview",
+      "display_name": "Gemini 3.1 Flash Lite",
+      "name": "models/gemini-3.1-flash-lite",
       "version": "3.1",
       "description": "Our smallest and most cost effective model, built for at scale usage.",
       "inputTokenLimit": 1048576,