Use TaskCompletionSource for thread-safe callback wait in C# cancel test

Addresses review feedback: the previous poll read observedRequest (written by the callback thread) from the test continuation without synchronization. Switch to the TaskCompletionSource pattern already used by the direct-RPC test in this file, so the callback result is handed off safely and awaited with a timeout.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: f7849882-a2a2-4587-a602-da7718889a8c

Author: stephentoub

dotnet/test/E2E/McpOAuthE2ETests.cs

@@ -204,13 +204,13 @@ public async Task Should_Cancel_Pending_MCP_OAuth_Request()
     {
         await using var oauthServer = await OAuthMcpServer.StartAsync(ExpectedToken);
         var serverName = "oauth-cancelled-mcp";
-        McpAuthContext? observedRequest = null;
+        var authRequest = new TaskCompletionSource<McpAuthContext>(TaskCreationOptions.RunContinuationsAsynchronously);
 
         await using var session = await CreateSessionAsync(new SessionConfig
         {
             OnMcpAuthRequest = request =>
             {
-                observedRequest = request;
+                authRequest.TrySetResult(request);
                 return Task.FromResult<McpAuthResult?>(McpAuthResult.Cancel());
             },
             McpServers = new Dictionary<string, McpServerConfig>
@@ -228,19 +228,13 @@ public async Task Should_Cancel_Pending_MCP_OAuth_Request()
         // The MCP connection is kicked off by session.create, but the SDK only registers its
         // `mcp.oauth_required` event interest once create returns. If the server's initial 401
         // wins that race, the runtime records `needs-auth` WITHOUT invoking the host callback,
-        // so `observedRequest` is briefly null even after `needs-auth` is observed. A later
-        // auth retry (now that interest is registered) invokes the callback with the same
-        // `Initial` reason. Wait for the callback rather than sampling it the instant
+        // so the callback fires only on a later auth retry (now that interest is registered),
+        // with the same `Initial` reason. Await the callback rather than sampling it the instant
         // `needs-auth` first appears, which is what made this test flaky.
-        await TestHelper.WaitForConditionAsync(
-            () => Task.FromResult(observedRequest is not null),
-            timeout: TimeSpan.FromSeconds(60),
-            pollInterval: TimeSpan.FromMilliseconds(200),
-            timeoutMessage: $"{serverName} OAuth request reaching the host callback");
+        var observedRequest = await authRequest.Task.WaitAsync(TimeSpan.FromSeconds(60));
 
-        Assert.NotNull(observedRequest);
-        Assert.NotEmpty(observedRequest!.RequestId);
-        Assert.Equal(serverName, observedRequest!.ServerName);
+        Assert.NotEmpty(observedRequest.RequestId);
+        Assert.Equal(serverName, observedRequest.ServerName);
         Assert.Equal(McpOauthRequestReason.Initial, observedRequest.Reason);
     }