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>

samples/TestServerWithHosting/Tools/SampleLlmTool.cs

@@ -12,7 +12,7 @@ public sealed class SampleLlmTool
 {
     [McpServerTool(Name = "sampleLLM"), Description("Samples from an LLM using MCP's sampling feature")]
     public static async Task<string> SampleLLM(
-        IMcpServer thisServer,
+        McpServer thisServer,
         [Description("The prompt to send to the LLM")] string prompt,
         [Description("Maximum number of tokens to generate")] int maxTokens,
         CancellationToken cancellationToken)