refactor: improve MCP client timeout handling and reactive testing

- Add configurable initialization timeout separate from request timeout
- Rename ServletSse* test classes to HttpSse* for better naming consistency
- Replace direct .block() calls with StepVerifier for better reactive testing
- Change ping() method to return Mono instead of Mono
- Improve error handling and reactive programming patterns throughout tests
- Chain reactive operations for cleaner test flow

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