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/class-diagrams.puml

@@ -1,25 +1,18 @@
 @startuml Core Components
 
 interface McpTransport {
-  +void start()
+  +Mono<Void> connect(Function<Mono<JSONRPCMessage>, 
+                       Mono<JSONRPCMessage>> handler)
+  +Mono<Void> sendMessage()
   +void close()
   +Mono<Void> closeGracefully()
-  +void setInboudMessageHandler()
-  +void setInboundErrorHandler()
-  +Mono<Void> sendMessage()
-}
-
-abstract class AbstractMcpTransport {
-  #ObjectMapper objectMapper
-  #getInboundSink()
-  #getOutboundSink()
-  #getErrorSink()
 }
 
 interface McpSession {
-  +<T> Mono<T> sendRequest()
-  +Mono<Void> sendNotification()
+  +<T> Mono<T> sendRequest(String method, Object requestParams, TypeReference<T> typeRef)
+  +Mono<Void> sendNotification(String method, Map<String, Object> params)
   +Mono<Void> closeGracefully()
+  +close()
 }
 
 class DefaultMcpSession {
@@ -52,12 +45,12 @@ class McpSyncClient {
   +GetPromptResult getPrompt()
 }
 
-class StdioServerTransport {
+class StdioClientTransport {
   -Process serverProcess
   -ServerParameters parameters
 }
 
-class SseServerTransport {
+class SseClientTransport {
   -WebClient webClient
 }
 
@@ -74,11 +67,10 @@ class McpError {
   -String message
 }
 
-McpTransport <|.. AbstractMcpTransport
-AbstractMcpTransport <|-- StdioServerTransport
-AbstractMcpTransport <|-- SseServerTransport
+McpTransport <|-- StdioClientTransport
+McpTransport <|-- SseClientTransport
 McpSession <|-- DefaultMcpSession
-DefaultMcpSession <|-- McpAsyncClient
+DefaultMcpSession --o McpAsyncClient
 McpClient ..> McpAsyncClient : creates
 McpClient ..> McpSyncClient : creates
 McpSyncClient o-- McpAsyncClient