Improve testing for StreamableHttpHandler and IdleTrackingBackgroundService (#345)

* Add idle session pruning tests
* Remove redundant CTS from StreamableHttpHandler
  * StreamableHttpServerTransport.HandlePostRequest already creates the equivelant CTS
    which DeleteRequest_CompletesSession_WhichCancelsLongRunningToolCalls verifies
* Fix accept header validation for GET
* Use InvalidOperationException for transport-not-connected errors in doc comments
* Fix flaky Cancellation_ThrowsCancellationException test

tests/Common/Utils/MockLoggerProvider.cs

@@ -0,0 +1,32 @@
+using Microsoft.Extensions.Logging;
+using System.Collections.Concurrent;
+
+namespace ModelContextProtocol.Tests.Utils;
+
+public class MockLoggerProvider() : ILoggerProvider
+{
+    public ConcurrentQueue<(string Category, LogLevel LogLevel, string Message, Exception? Exception)> LogMessages { get; } = [];
+
+    public ILogger CreateLogger(string categoryName)
+    {
+        return new MockLogger(this, categoryName);
+    }
+
+    public void Dispose()
+    {
+    }
+
+    private class MockLogger(MockLoggerProvider mockProvider, string category) : ILogger
+    {
+        public void Log<TState>(
+            LogLevel logLevel, EventId eventId, TState state, Exception? exception, Func<TState, Exception?, string> formatter)
+        {
+            mockProvider.LogMessages.Enqueue((category, logLevel, formatter(state, exception), exception));
+        }
+
+        public bool IsEnabled(LogLevel logLevel) => true;
+
+        // The MockLoggerProvider is a convenient NoopDisposable
+        public IDisposable BeginScope<TState>(TState state) where TState : notnull => mockProvider;
+    }
+}