fix: improve error handling in HttpClientSseClientTransport and add test

- Include response body in error messages for better debugging
- Change sendHttpPost return type from HttpResponse<Void> to HttpResponse<String>
- Use BodyHandlers.ofString() instead of discarding response body
- Add testErrorOnBogusMessage test to verify error handling for invalid JSON-RPC messages
- Test validates that malformed messages return 400 status with descriptive error message

Replace #361

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

Co-authored-by: jkma <jkma@trip.com>