refactor(mcp): replace HttpServletSseServerTransport with provider-based implementation

Introduces HttpServletSseServerTransportProvider as a replacement for HttpServletSseServerTransport,
following the provider pattern used by other transport implementations. The new implementation offers
the same functionality but with a more consistent API aligned with the McpServerTransportProvider interface.

- Mark HttpServletSseServerTransport as @deprecated (to be removed in 0.9.0)
- Add new HttpServletSseServerTransportProvider implementation
- Update test classes to use the new provider-based implementation
- Add separate test classes for deprecated implementation

Signed-off-by: Christian Tzolov <[email protected]>

Author: tzolov

mcp/src/main/java/io/modelcontextprotocol/server/transport/HttpServletSseServerTransport.java

@@ -32,6 +32,9 @@
  * specification. This implementation provides similar functionality to
  * WebFluxSseServerTransport but uses the traditional Servlet API instead of WebFlux.
  *
+ * @deprecated This class will be removed in 0.9.0. Use
+ * {@link HttpServletSseServerTransportProvider}.
+ *
  * <p>
  * The transport handles two types of endpoints:
  * <ul>
@@ -48,14 +51,14 @@
  * <li>Graceful shutdown support</li>
  * <li>Error handling and response formatting</li>
  * </ul>
- *
  * @author Christian Tzolov
  * @author Alexandros Pappas
  * @see ServerMcpTransport
  * @see HttpServlet
  */
 
 @WebServlet(asyncSupported = true)
+@Deprecated
 public class HttpServletSseServerTransport extends HttpServlet implements ServerMcpTransport {
 
 	/** Logger for this class */