Replace Task.WhenAny timeout pattern with Task.WaitAsync

[Copilot]

Refactors dispose timeout tests to use Task.WaitAsync instead of Task.WhenAny with Task.Delay, per review feedback on #1252.

Changes

• Replaced verbose timeout pattern in 3 test methods across StreamableHttpClientConformanceTests and MapMcpStreamableHttpTests
• Eliminated manual timeout comparison and assertions - Task.WaitAsync throws TimeoutException automatically

Before

var disposeTask = client.DisposeAsync().AsTask();
var completedTask = await Task.WhenAny(disposeTask, Task.Delay(TimeSpan.FromSeconds(10), cancellationToken));
Assert.True(disposeTask == completedTask, "DisposeAsync hung");
await disposeTask;

After

await client.DisposeAsync().AsTask().WaitAsync(TimeSpan.FromSeconds(10), cancellationToken);

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