Clean up namespaces (#410)

No meaningful code changes, just lots of namespace shuffling to have fewer namespaces and to put things in better namespaces.

src/ModelContextProtocol/SemaphoreSlimExtensions.cs

@@ -0,0 +1,40 @@
+namespace ModelContextProtocol;
+
+internal static class SynchronizationExtensions
+{
+    /// <summary>
+    /// Asynchronously acquires a lock on the semaphore and returns a disposable object that releases the lock when disposed.
+    /// </summary>
+    /// <param name="semaphore">The semaphore to acquire a lock on.</param>
+    /// <param name="cancellationToken">A cancellation token to observe while waiting for the semaphore.</param>
+    /// <returns>A disposable <see cref="Releaser"/> that releases the semaphore when disposed.</returns>
+    /// <remarks>
+    /// This extension method provides a convenient pattern for using a semaphore in asynchronous code,
+    /// similar to how the `lock` statement is used in synchronous code.
+    /// </remarks>
+    /// <exception cref="OperationCanceledException">The <paramref name="cancellationToken"/> was canceled.</exception>
+    public static async ValueTask<Releaser> LockAsync(this SemaphoreSlim semaphore, CancellationToken cancellationToken = default)
+    {
+        await semaphore.WaitAsync(cancellationToken).ConfigureAwait(false);
+        return new(semaphore);
+    }
+
+    /// <summary>
+    /// A disposable struct that releases a semaphore when disposed.
+    /// </summary>
+    /// <remarks>
+    /// This struct is used with the <see cref="LockAsync"/> extension method to provide
+    /// a using-pattern for semaphore locking, similar to lock statements.
+    /// </remarks>
+    public readonly struct Releaser(SemaphoreSlim semaphore) : IDisposable
+    {
+        /// <summary>
+        /// Releases the semaphore.
+        /// </summary>
+        /// <remarks>
+        /// This method is called automatically when the <see cref="Releaser"/> goes out of scope
+        /// in a using statement or expression.
+        /// </remarks>
+        public void Dispose() => semaphore.Release();
+    }
+}