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

README.md

@@ -12,7 +12,7 @@ Spring AI MCP is an experimental project that provides Java and Spring Framework
 
 The project consists of two main modules:
 
-### spring-ai-mcp-core
+### [spring-ai-mcp-core](./spring-ai-mcp-core/README.md)
 
 The core module provides a Java implementation of the Model Context Protocol specification. It includes:
 - Synchronous and asynchronous client implementations
@@ -20,7 +20,9 @@ The core module provides a Java implementation of the Model Context Protocol spe
 - Stdio-based server transport
 - Reactive programming support using Project Reactor
 
-### spring-ai-mcp-spring
+[find more](./spring-ai-mcp-core/README.md)
+
+### [spring-ai-mcp-spring](./spring-ai-mcp-spring/README.md)
 
 The Spring integration module provides Spring-specific functionality:
 - Integration with Spring AI's function calling system