Fix Windows build error due to implicit cast of uint64_t to WebTransportStreamError which is uint8_t.

Add explicit cast to avoid the build error on Windows.

PiperOrigin-RevId: 506717737

quiche/quic/core/http/web_transport_http3.cc

diff --git a/quiche/quic/core/http/web_transport_http3.cc b/quiche/quic/core/http/web_transport_http3.cc
index 91372d5..1896266 100644
--- a/quiche/quic/core/http/web_transport_http3.cc
+++ b/quiche/quic/core/http/web_transport_http3.cc
@@ -440,7 +440,7 @@
   uint64_t shifted = http3_error_code - kWebTransportMappedErrorCodeFirst;
   uint64_t result = shifted - shifted / 0x1f;
   QUICHE_DCHECK_LE(result, std::numeric_limits<uint8_t>::max());
-  return result;
+  return static_cast<WebTransportStreamError>(result);
 }
 
 WebTransportStreamError Http3ErrorToWebTransportOrDefault(