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>

docs/concepts/elicitation/samples/server/Tools/InteractiveTools.cs

@@ -13,7 +13,7 @@ public sealed class InteractiveTools
     // <snippet_GuessTheNumber>
     [McpServerTool, Description("A simple game where the user has to guess a number between 1 and 10.")]
     public async Task<string> GuessTheNumber(
-        IMcpServer server, // Get the McpServer from DI container
+        McpServer server, // Get the McpServer from DI container
         CancellationToken token
     )
     {