fix(antigravity): correctly mark credits exhausted on "Resource has been exhausted" 429

shouldMarkCreditsExhausted was blocked by isURLLevelRateLimit check when
credit overages retry returned "Resource has been exhausted (e.g. check quota).",
causing credits to never be marked as exhausted. This led to an infinite loop
where each request injected credits, bypassed model rate limits, and failed again.

- Remove isURLLevelRateLimit guard from shouldMarkCreditsExhausted (only called
  for credit retry responses — if credits retry fails, mark exhausted)
- Add "resource has been exhausted" to creditsExhaustedKeywords
- Update tests to match corrected behavior

backend/internal/service/antigravity_credits_overages_test.go

@@ -406,10 +406,16 @@ func TestShouldMarkCreditsExhausted(t *testing.T) {
 		require.False(t, shouldMarkCreditsExhausted(resp, []byte(`{"error":"Insufficient credits"}`), nil))
 	})
 
-	t.Run("URL 级限流不标记", func(t *testing.T) {
+	t.Run("Resource has been exhausted 应标记为积分耗尽", func(t *testing.T) {
 		resp := &http.Response{StatusCode: http.StatusTooManyRequests}
 		body := []byte(`{"error":{"message":"Resource has been exhausted"}}`)
-		require.False(t, shouldMarkCreditsExhausted(resp, body, nil))
+		require.True(t, shouldMarkCreditsExhausted(resp, body, nil))
+	})
+
+	t.Run("Resource has been exhausted (check quota) 完整格式应标记", func(t *testing.T) {
+		resp := &http.Response{StatusCode: http.StatusTooManyRequests}
+		body := []byte(`{"error":{"code":429,"message":"Resource has been exhausted (e.g. check quota).","status":"RESOURCE_EXHAUSTED"}}`)
+		require.True(t, shouldMarkCreditsExhausted(resp, body, nil))
 	})
 
 	t.Run("结构化限流不标记", func(t *testing.T) {