Fix ObjectDisposedException by awaiting handler tasks before session disposal

[Varun6578]

Summary

Fixes #1269

When using HTTP transport with Stateless = true, if the client closes the connection (e.g., OpenAI times out on a long-running tool), the ASP.NET Core request scope is disposed while tool handlers are still executing, causing ObjectDisposedException.

Root Cause

In McpSessionHandler.ProcessMessagesCoreAsync(), message handlers were launched as fire-and-forget tasks (_ = ProcessMessageAsync()). These tasks were not tracked or awaited.

When McpSessionHandler.DisposeAsync() cancelled the message processing CTS and awaited _messageProcessingTask, the channel reader loop exited immediately but the fire-and-forget handler tasks continued running. The session disposal then completed, ASP.NET Core disposed the request service scope, and the still-running handlers got ObjectDisposedException when resolving scoped services.

Fix

• Track all handler tasks in a List<Task> instead of discarding them
• await Task.WhenAll(pendingHandlerTasks) in the finally block of ProcessMessagesCoreAsync before the method returns
• Periodically prune completed tasks to avoid unbounded list growth
• Added a regression test verifying scoped services remain accessible throughout a delayed handler's lifetime

Test Results

• Build: 0 warnings, 0 errors
• Core tests: 1838 passed (net10.0), 1838 passed (net9.0), 1654 passed (net472)
• ASP.NET Core tests: 302 passed (2 pre-existing conformance failures unrelated to this change)