feat: Add kotlin-sdk-testing module with channel-based transport implementation (#505)

# feat: Add kotlin-sdk-testing module with channel-based transport
implementation

- Introduced `kotlin-sdk-testing` module for testing utilities in the
MCP Kotlin SDK.
- Implemented `ChannelTransport`, providing JSON-RPC transport via
Kotlin coroutines channels.

<!-- Provide a brief summary of your changes -->

## Motivation and Context
This ChannelTransport can be used for testing Server implementation
without using real transports.

## How Has This Been Tested?
Unit test

## Breaking Changes
No

## Types of changes

- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [x] I have added or updated documentation as needed

README.md

@@ -46,6 +46,7 @@ standardized protocol interface.
   * [Streamable HTTP Transport](#streamable-http-transport)
   * [SSE Transport](#sse-transport)
   * [WebSocket Transport](#websocket-transport)
+  * [ChannelTransport (testing)](#channeltransport-testing)
 * [Connecting your server](#connecting-your-server)
 * [Examples](#examples)
 * [Documentation](#documentation)
@@ -774,6 +775,12 @@ SSE Ktor plugin when you need drop-in compatibility, but prefer Streamable HTTP
 `WebSocketClientTransport` plus the matching server utilities provide full-duplex, low-latency connections—useful when
 you expect lots of notifications or long-running sessions behind a reverse proxy that already terminates WebSockets.
 
+### ChannelTransport (testing)
+
+`ChannelTransport` provides a simple, non-networked transport for testing and local development.
+It uses Kotlin coroutines channels to provide a full-duplex connection between a client and server,
+allowing for easy testing of MCP functionality without the need for network setup.
+
 ## Connecting your server
 
 1. Start a sample HTTP server on port 3000: