feat(mcp): Add support for custom context paths in HTTP Servlet SSE server transport

Enhance HttpServletSseServerTransportProvider to support deployment under non-root context paths by:

- Adding baseUrl field and DEFAULT_BASE_URL constant
- Creating new constructor that accepts a baseUrl parameter
- Extending Builder with baseUrl configuration method
- Prepending baseUrl to message endpoint in SSE events
- Add HttpServletSseServerCustomContextPathTests to verify custom context path functionality
- Extract common Tomcat server setup code to TomcatTestUtil for test reuse

Related to #79

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>