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/gateway_service.go

@@ -2587,6 +2587,9 @@ func (s *GatewayService) isModelSupportedByAccountWithContext(ctx context.Contex
 		// 应用 thinking 后缀后检查最终模型是否在账号映射中
 		if enabled, ok := ctx.Value(ctxkey.ThinkingEnabled).(bool); ok {
 			finalModel := applyThinkingModelSuffix(mapped, enabled)
+			if finalModel == mapped {
+				return true // thinking 后缀未改变模型名，映射已通过
+			}
 			return account.IsModelSupported(finalModel)
 		}
 		return true