perf(server): skip IdleTrackingBackgroundService timer in stateless mode

[MukundaKatta]

Why

Closes #1515.

IdleTrackingBackgroundService runs a PeriodicTimer every 5 seconds to prune idle sessions and enforce MaxIdleSessionCount. In stateless mode (HttpServerTransportOptions.Stateless = true), the StatefulSessionManager is never populated — StreamableHttpSession returns early without storing the session — so there is nothing to prune.

Running the timer regardless wastes CPU on every server instance and clutters shutdown paths with a DisposeAllSessionsAsync that has nothing to dispose. With 1.2.0 leaning into stateless-by-default, this is wasted work on most servers.

What

• Override BackgroundService.StartAsync to early-return Task.CompletedTask when _options.Value.Stateless is true. The timer loop in ExecuteAsync is never started, and the BackgroundService.ExecuteTask stays null. This was the fix proposed by the issue author and matches the architectural seam exactly: options have been bound by the time StartAsync runs, so we don't need to re-plumb DI.

Tested

• Added two unit tests in HttpMcpServerBuilderExtensionsTests.cs:
  • IdleTrackingBackgroundService_DoesNotStartTimer_WhenStateless asserts ExecuteTask is null after StartAsync when Stateless = true.
  • IdleTrackingBackgroundService_StartsTimer_WhenStateful asserts the inverse for the default stateful mode.
• Tests look the service up by type name from IServiceProvider.GetServices<IHostedService>() since IdleTrackingBackgroundService is internal sealed and the test project has no InternalsVisibleTo.