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

---------

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

samples/EverythingServer/Tools/LongRunningTool.cs

@@ -10,7 +10,7 @@ public class LongRunningTool
 {
     [McpServerTool(Name = "longRunningOperation"), Description("Demonstrates a long running operation with progress updates")]
     public static async Task<string> LongRunningOperation(
-        IMcpServer server,
+        McpServer server,
         RequestContext<CallToolRequestParams> context,
         int duration = 10,
         int steps = 5)

samples/EverythingServer/Tools/SampleLlmTool.cs

@@ -9,7 +9,7 @@ public class SampleLlmTool
 {
     [McpServerTool(Name = "sampleLLM"), Description("Samples from an LLM using MCP's sampling feature")]
     public static async Task<string> SampleLLM(
-        IMcpServer server,
+        McpServer server,
         [Description("The prompt to send to the LLM")] string prompt,
         [Description("Maximum number of tokens to generate")] int maxTokens,
         CancellationToken cancellationToken)