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.SSE.TRANSPORT.md

@@ -2,7 +2,7 @@
 
 ## Overview
 
-The `SseServerTransport` class implements the Model Context Protocol (MCP) HTTP with SSE transport specification, providing a bidirectional communication channel between clients and servers. This implementation is part of the Spring AI MCP Core library and follows the [MCP HTTP with SSE Transport Specification](https://spec.modelcontextprotocol.io/specification/basic/transports/#http-with-sse).
+The `SseClientTransport` class implements the Model Context Protocol (MCP) HTTP with SSE transport specification, providing a bidirectional communication channel between clients and servers. This implementation is part of the Spring AI MCP Core library and follows the [MCP HTTP with SSE Transport Specification](https://spec.modelcontextprotocol.io/specification/basic/transports/#http-with-sse).
 
 ## Key Features
 
@@ -53,7 +53,7 @@ WebClient.Builder webClientBuilder = WebClient.builder()
     .baseUrl("http://localhost:3001");
 
 // Initialize transport
-SseServerTransport transport = new SseServerTransport(webClientBuilder);
+SseClientTransport transport = new SseClientTransport(webClientBuilder);
 
 // Create a JSON-RPC request
 JSONRPCRequest request = new JSONRPCRequest(
@@ -141,4 +141,4 @@ The implementation includes comprehensive tests covering:
 - Shutdown procedures
 - Edge cases and failure scenarios
 
-For detailed test examples, see `SseServerTransportTests.java`.
+For detailed test examples, see `SseClientTransportTests.java`.