fix(antigravity): support upstream accounts and custom model_mapping in scheduling

- GetAccessToken: add upstream branch to read api_key from credentials
- shouldTriggerAntigravitySmartRetry: relax check from IsOAuth to Platform-based
- isModelSupportedByAccount/WithContext: replace IsAntigravityModelSupported
  whitelist with mapAntigravityModel for unified scheduling/forwarding logic
- mapAntigravityModel: fix edge case where wildcard target equals request model
- Update tests for new behavior and add custom model_mapping test cases

backend/internal/service/antigravity_rate_limit_test.go

@@ -524,8 +524,8 @@ func TestParseAntigravitySmartRetryInfo(t *testing.T) {
 }
 
 func TestShouldTriggerAntigravitySmartRetry(t *testing.T) {
-	oauthAccount := &Account{Type: AccountTypeOAuth}
-	setupTokenAccount := &Account{Type: AccountTypeSetupToken}
+	oauthAccount := &Account{Type: AccountTypeOAuth, Platform: PlatformAntigravity}
+	setupTokenAccount := &Account{Type: AccountTypeSetupToken, Platform: PlatformAntigravity}
 	apiKeyAccount := &Account{Type: AccountTypeAPIKey}
 
 	tests := []struct {