feat: implement Streamable HTTP HttpClient transport and improve error handling (#337)

- Add HttpClientStreamableHttpTransport implementing the 2025-03-26 MCP specification
- Add ResponseSubscribers utility for handling different HTTP response body types
- Refactor HttpClientSseClientTransport to use reactive streams instead of FlowSseClient
  - Remove FlowSseClient in favor of reactive stream patterns
  - Use Disposable-based connection management instead of CountDownLatch
  - Replace message endpoint discovery with Sinks.One approach
- Improve error handling by replacing doOnError/onErrorResume with onErrorComplete
- Add test coverage for new Streamable HTTP transport
- Add resiliency tests for connection handling and session invalidation

BREAKING CHANGE: FlowSseClient has been removed in favor of reactive streams

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

Co-authored-by: Dariusz Jędrzejczyk <dariusz.jedrzejczyk@broadcom.com>