custom SSE + HTTP POST transport

[ngxson]

ref comment: ggml-org#18334 (comment)

TL;DR:

• Completely removed websocket
• Replaced with SSE + HTTP POST request as mentioned earlier
• Transport served under /mcp endpoint (GET + POST)
• Use a custom transport class on webui to bypass the auth logic - we are running on the same host with the webui, not an external server, there is no need for CORS or auth
• The code can be reduced even further

Benefits are:

• Users already serving llama.cpp via reverse proxy have don't have to change anything - it just works
• SSL is automatically handled by httplib
• C++ code is much simpler than websocket

Tested with python tool:

[ochafik]

Thanks @ngxson !!
For SSE we'd need to proxy get /message too.

What issues did you face w/ the stock transports / why the new one?

Also we should be able to add auth headers (if needed for server started w/ --api-key) by giving requestInit.headers to the transport ctor.