refactor: Add HttpServletSseServerTransport for generic Servlet integration

Introduces a new Server-Sent Events (SSE) Server transport implementation
based on the Servlet API that enables MCP integration with any Java HTTP
server application supporting Servlets.

This implementation complements the existing WebFlux and WebMvc transports
by providing a generic Servlet-based option that works with any Java web
server, not just Spring applications.

Key changes:

- Add HttpServletSseServerTransport with async Servlet 6.0 support
- Add integration tests with embedded Tomcat
- Add servlet dependencies with externalized versions:
  - jakarta.servlet-api 6.1.0 (provided)
  - tomcat-embed-core 11.0.2 (test)
  - tomcat-embed-websocket 11.0.2 (test)

Resolves #61