feat: add SSE transport and improve documentation

- Add Server-Sent Events (SSE) transport implementation with test coverage
- Add detailed documentation for both SSE and STDIO transports
- Extract common test logic into abstract base classes
- Add testcontainers support for SSE integration tests
- Replace System.out with SLF4J logging
- Externalize dependency versions to properties
- Implement endpoint event handling per MCP spec
- Add message endpoint sink for reliable message routing
- Improve error handling and graceful shutdown
- Update core module README with expanded features and examples
- Add dependency version properties and logging configuration
- Update architecture and class diagrams

Resolves #4

spring-ai-mcp-core/docs/class-diagrams.puml

@@ -57,6 +57,10 @@ class StdioServerTransport {
   -ServerParameters parameters
 }
 
+class SseServerTransport {
+  -WebClient webClient
+}
+
 class McpSchema {
   +class ErrorCodes
   +interface Request
@@ -72,6 +76,7 @@ class McpError {
 
 McpTransport <|.. AbstractMcpTransport
 AbstractMcpTransport <|-- StdioServerTransport
+AbstractMcpTransport <|-- SseServerTransport
 McpSession <|-- DefaultMcpSession
 DefaultMcpSession <|-- McpAsyncClient
 McpClient ..> McpAsyncClient : creates