Flaky multiple HTTP/2 connection tests read full request body (dotnet…

…#40844)

There are 2 flaky HTTP/2 tests verifying multiple connections feature which are randomly failing on CI, but not locally.
- Http2_MultipleConnectionsEnabled_ConnectionLimitNotReached_ConcurrentRequestsSuccessfullyHandled
- Http2_MultipleConnectionsEnabled_IdleConnectionTimeoutExpired_ConnectionRemovedAndNewCreated

It seems the failure is caused by not reading the request body. In current implementation, `Http2LoopbackServer` read only HEADERS frame via `ReadRequestHeaderAsync` and then immediately sends response. However, the client firstly completely sends headers and body and only then starting reading a response. Thus, it seems to get blocked sometimes if the server didn't read the full body. This PR fixes this by calling `ReadAndParseRequestHeaderAsync` instead of `ReadRequestHeaderAsync`.

Fixes dotnet#40436
Fixes dotnet#40115

src/libraries/System.Net.Http/tests/FunctionalTests/SocketsHttpHandlerTest.cs

@@ -2303,23 +2303,24 @@ private static async Task<Http2LoopbackConnection> GetConnection(Http2LoopbackSe
 
         private async Task<(int Count, int LastStreamId)> HandleAllPendingRequests(Http2LoopbackConnection connection, int totalRequestCount)
         {
-            int streamId = -1;
+            int lastStreamId = -1;
             for (int i = 0; i < totalRequestCount; i++)
             {
                 try
                 {
                     // Exact number of requests sent over the given connection is unknown,
                     // so we keep reading headers and sending response while there are available requests.
-                    streamId = await connection.ReadRequestHeaderAsync().ConfigureAwait(false);
+                    (int streamId, _) = await connection.ReadAndParseRequestHeaderAsync().ConfigureAwait(false);
                     await connection.SendDefaultResponseAsync(streamId).ConfigureAwait(false);
+                    lastStreamId = streamId;
                 }
                 catch (OperationCanceledException)
                 {
-                    return (i, streamId);
+                    return (i, lastStreamId);
                 }
             }
 
-            return (totalRequestCount, streamId);
+            return (totalRequestCount, lastStreamId);
         }
 
         private async Task<List<int>> AcceptRequests(Http2LoopbackConnection connection, int maxRequests = int.MaxValue)
@@ -2329,7 +2330,8 @@ private async Task<List<int>> AcceptRequests(Http2LoopbackConnection connection,
             {
                 try
                 {
-                    streamIds.Add(await connection.ReadRequestHeaderAsync().ConfigureAwait(false));
+                    (int streamId, _) = await connection.ReadAndParseRequestHeaderAsync().ConfigureAwait(false);
+                    streamIds.Add(streamId);
                 }
                 catch (OperationCanceledException)
                 {