fix(antigravity): fast-fail on proxy unavailable, temp-unschedule account

## Problem

When a proxy is unreachable, token refresh retries up to 4 times with
30s timeout each, causing requests to hang for ~2 minutes before
failing with a generic 502 error. The failed account is not marked,
so subsequent requests keep hitting it.

## Changes

### Proxy connection fast-fail
- Set TCP dial timeout to 5s and TLS handshake timeout to 5s on
  antigravity client, so proxy connectivity issues fail within 5s
  instead of 30s
- Reduce overall HTTP client timeout from 30s to 10s
- Export `IsConnectionError` for service-layer use
- Detect proxy connection errors in `RefreshToken` and return
  immediately with "proxy unavailable" error (no retries)

### Token refresh temp-unschedulable
- Add 8s context timeout for token refresh on request path
- Mark account as temp-unschedulable for 10min when refresh fails
  (both background `TokenRefreshService` and request-path
  `GetAccessToken`)
- Sync temp-unschedulable state to Redis cache for immediate
  scheduler effect
- Inject `TempUnschedCache` into `AntigravityTokenProvider`

### Account failover
- Return `UpstreamFailoverError` on `GetAccessToken` failure in
  `Forward`/`ForwardGemini` to trigger handler-level account switch
  instead of returning 502 directly

### Proxy probe alignment
- Apply same 5s dial/TLS timeout to shared `httpclient` pool
- Reduce proxy probe timeout from 30s to 10s

backend/internal/service/antigravity_gateway_service.go

@@ -1359,7 +1359,10 @@ func (s *AntigravityGatewayService) Forward(ctx context.Context, c *gin.Context,
 	}
 	accessToken, err := s.tokenProvider.GetAccessToken(ctx, account)
 	if err != nil {
-		return nil, s.writeClaudeError(c, http.StatusBadGateway, "authentication_error", "Failed to get upstream access token")
+		return nil, &UpstreamFailoverError{
+			StatusCode:   http.StatusBadGateway,
+			ResponseBody: []byte(`{"error":{"type":"authentication_error","message":"Failed to get upstream access token"},"type":"error"}`),
+		}
 	}
 
 	// 获取 project_id（部分账户类型可能没有）
@@ -2101,7 +2104,10 @@ func (s *AntigravityGatewayService) ForwardGemini(ctx context.Context, c *gin.Co
 	}
 	accessToken, err := s.tokenProvider.GetAccessToken(ctx, account)
 	if err != nil {
-		return nil, s.writeGoogleError(c, http.StatusBadGateway, "Failed to get upstream access token")
+		return nil, &UpstreamFailoverError{
+			StatusCode:   http.StatusBadGateway,
+			ResponseBody: []byte(`{"error":{"message":"Failed to get upstream access token","status":"UNAVAILABLE"}}`),
+		}
 	}
 
 	// 获取 project_id（部分账户类型可能没有）