Restructure IMcpEndpoint, IMcpClient, and IMcpServer (#723)

* Remove `McpEndpoint` & `IMcpClient`

* Revise naming, add abstract classes

* Fix missing renames

* Move extension methods to instance methods

* Add back old API, but obsoleted

* Add back obsoleted factory classes

* Move deprecation attributes

* `SseClientTransport` -> `HttpClientTransport`

* Update src/ModelContextProtocol.Core/Server/AugmentedServiceProvider.cs

Co-authored-by: Stephen Halter <halter73@gmail.com>

* PR feedback

* PR feedback

* Add back dispose lock

* Fix docs projects

* PR feedback

* Undo `docs/*.md` changes

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Co-authored-by: Stephen Halter <halter73@gmail.com>

src/ModelContextProtocol/IMcpServerBuilder.cs

@@ -3,7 +3,7 @@ using ModelContextProtocol.Server;
 namespace Microsoft.Extensions.DependencyInjection;
 
 /// <summary>
-/// Provides a builder for configuring <see cref="IMcpServer"/> instances.
+/// Provides a builder for configuring <see cref="McpServer"/> instances.
 /// </summary>
 /// <remarks>
 /// <para>