Refactor the mcp-core module

- The core module has been refactored to remove unnecessary abstraction
  between the SSE transport and the STDIO transport.
- Tests have been improved and no longer complain about multiple
  subscriptions to single-subscriber Sink.
- SSE transport layer is now assuming non-blocking execution and no longer
  makes thread hops when not needed.
- Decouple ObjectMapper and error handling from AbstractMcpTransport
  and remove later.

Additional improvements:

- Rename transport classes to better reflect client-side role (*ServerTransport -> *ClientTransport)
- Introduce fluent builder pattern for client configuration
- Simplify DefaultMcpSession by removing ObjectMapper dependency
- Move unmarshal responsibility to transport layer
- Update documentation and diagrams to reflect new architecture
- Clean up test implementations

spring-ai-mcp-core/docs/CLIENT.STDIO.TRANSPORT.md

@@ -1,6 +1,6 @@
-# StdioServerTransport
+# StdioClientTransport
 
-The `StdioServerTransport` class implements the [MCP Stdio Transport](https://spec.modelcontextprotocol.io/specification/basic/transports/#stdio) specification, providing communication with MCP servers over standard input/output streams.
+The `StdioClientTransport` class implements the [MCP Stdio Transport](https://spec.modelcontextprotocol.io/specification/basic/transports/#stdio) specification, providing communication with MCP servers over standard input/output streams.
 
 ## Overview
 
@@ -84,7 +84,7 @@ ServerParameters params = ServerParameters.builder()
     .build();
 
 // Create transport
-StdioServerTransport transport = new StdioServerTransport(params);
+StdioClientTransport transport = new StdioClientTransport(params);
 
 // Start transport
 transport.start();