fix: 修复 CI 集成测试因 context deadline exceeded 未被跳过而失败

skipIfExternalServiceUnavailable 检查了 "timeout" 但 Go 的
context.DeadlineExceeded 错误信息是 "context deadline exceeded"，
不包含 "timeout" 子串，导致外部服务不可达时测试直接失败而非跳过。

backend/internal/pkg/tlsfingerprint/dialer_integration_test.go

@@ -30,7 +30,8 @@ func skipIfExternalServiceUnavailable(t *testing.T, err error) {
 			strings.Contains(errStr, "connection refused") ||
 			strings.Contains(errStr, "no such host") ||
 			strings.Contains(errStr, "network is unreachable") ||
-			strings.Contains(errStr, "timeout") {
+			strings.Contains(errStr, "timeout") ||
+			strings.Contains(errStr, "deadline exceeded") {
 			t.Skipf("skipping test: external service unavailable: %v", err)
 		}
 		t.Fatalf("failed to get fingerprint: %v", err)