test(antigravity): add missing unit tests for upstream and custom model_mapping

- Add GetAccessToken upstream branch tests (success/failure/empty/nil)
- Add mapAntigravityModel wildcard-target-equals-request edge case tests
- Add upstream account smart retry test case
- Add GeminiMessagesCompatService custom model_mapping and empty model tests

backend/internal/service/antigravity_rate_limit_test.go

@@ -526,6 +526,7 @@ func TestParseAntigravitySmartRetryInfo(t *testing.T) {
 func TestShouldTriggerAntigravitySmartRetry(t *testing.T) {
 	oauthAccount := &Account{Type: AccountTypeOAuth, Platform: PlatformAntigravity}
 	setupTokenAccount := &Account{Type: AccountTypeSetupToken, Platform: PlatformAntigravity}
+	upstreamAccount := &Account{Type: AccountTypeUpstream, Platform: PlatformAntigravity}
 	apiKeyAccount := &Account{Type: AccountTypeAPIKey}
 
 	tests := []struct {
@@ -587,6 +588,23 @@ func TestShouldTriggerAntigravitySmartRetry(t *testing.T) {
 			expectedShouldRateLimit: true,
 			modelName:               "claude-sonnet-4-5",
 		},
+		{
+			name:    "Upstream account with short delay - smart retry",
+			account: upstreamAccount,
+			body: `{
+				"error": {
+					"status": "RESOURCE_EXHAUSTED",
+					"details": [
+						{"@type": "type.googleapis.com/google.rpc.ErrorInfo", "metadata": {"model": "claude-sonnet-4-5"}, "reason": "RATE_LIMIT_EXCEEDED"},
+						{"@type": "type.googleapis.com/google.rpc.RetryInfo", "retryDelay": "2s"}
+					]
+				}
+			}`,
+			expectedShouldRetry:     true,
+			expectedShouldRateLimit: false,
+			minWait:                 2 * time.Second,
+			modelName:               "claude-sonnet-4-5",
+		},
 		{
 			name:    "API Key account - should not trigger",
 			account: apiKeyAccount,