Enforce SendPendingMessages in dispatch queue (#14487)

* Make ExperimentalFeatures overridable

* Enforce SendPendingMessages in dispatch queue

* Add WinUI 3 MSO UT packages lock file

* Change files

* Return if actual write fails

* Remove unrelated lock file

* Revert ExperimentalFeatures.props

Author: JunielKatarn

change/react-native-windows-2936f6eb-3bf6-498e-9099-202818e8cd17.json

@@ -0,0 +1,7 @@
+{
+  "type": "prerelease",
+  "comment": "Enforce SendPendingMessages in dispatch queue",
+  "packageName": "react-native-windows",
+  "email": "julio.rocha@microsoft.com",
+  "dependentChangeType": "patch"
+}

vnext/Shared/Networking/WinRTWebSocketResource.cpp

@@ -290,6 +290,10 @@ fire_and_forget WinRTWebSocketResource2::PerformWrite(string &&message, bool isB
 }
 
 IAsyncAction WinRTWebSocketResource2::SendPendingMessages() noexcept {
+  // Enforcing execution in the background queue.
+  // Awaiting of this coroutine will schedule its execution in the thread pool, ignoring the intended dispatch queue.
+  co_await resume_in_queue(m_backgroundQueue);
+
   auto self = shared_from_this();
 
   while (!self->m_outgoingMessages.empty()) {
@@ -334,6 +338,7 @@ IAsyncAction WinRTWebSocketResource2::SendPendingMessages() noexcept {
     auto result = async.ErrorCode();
     if (result < 0) {
       Fail(std::move(result), ErrorType::Send);
+      co_return;
     }
   }
 }