test: update gemini-2.5-pro mapping test case

Update test expectation to reflect new mapping:
gemini-2.5-pro -> gemini-3-pro-high

backend/internal/service/antigravity_model_mapping_test.go

@@ -142,10 +142,10 @@ func TestAntigravityGatewayService_GetMappedModel(t *testing.T) {
 			expected:       "gemini-2.5-flash",
 		},
 		{
-			name:           "Gemini透传 - gemini-2.5-pro",
+			name:           "Gemini映射 - gemini-2.5-pro → gemini-3-pro-high",
 			requestedModel: "gemini-2.5-pro",
 			accountMapping: nil,
-			expected:       "gemini-2.5-pro",
+			expected:       "gemini-3-pro-high",
 		},
 		{
 			name:           "Gemini透传 - gemini-future-model",