Transport-neutral MCP HTTP engine.

Holds the protocol logic for both the simple HTTP transport and the Streamable HTTP transport (POST/GET/DELETE/OPTIONS, SSE, sessions, CORS, Origin validation, authentication, the OAuth protected-resource-metadata endpoint and async tool calls) WITHOUT any dependency on a concrete HTTP server.

A binding (the built-in barrel_mcp_http_listener h1/h2 server, or an external adapter such as Livery's) reads the request line and body, then calls handle/6 with:

• Method — the request method binary (<<"POST">> …).
• Path — the request target (query string allowed; it is stripped here).
• Headers — a [{binary(), binary()}] proplist. Lookups are case-insensitive.
• Body — the full request body (<<>> when none).
• Responder — a map of I/O closures (see below).
• Config — the engine configuration (see the config() type).

The Responder abstracts response delivery so the engine never touches a socket:

   #{reply        => fun((Status, Headers, Body) -> ok),
     stream_start => fun((Status, Headers) -> ok),
     stream_chunk => fun((iodata()) -> ok | {error, term()}),
     stream_end   => fun(() -> ok)}

Headers passed to the closures is a [{binary(), binary()}] proplist (lowercase names). A streaming (SSE) response is stream_start then repeated stream_chunk then stream_end.

handle/6 runs in the calling (per-request) process. For a long-lived GET SSE stream it blocks in a receive loop until the session is terminated or the binding signals a client disconnect by sending the calling process the message mcp_disconnect.