Streamable HTTP transport for barrel_mcp_client.

Implements MCP's Streamable HTTP transport (2025-03-26 onward) on the client side:

• POST every request with Accept: application/json, text/event-stream. The server may answer with a single JSON envelope or with an SSE stream that interleaves server-initiated requests/notifications until the matching response arrives.
• GET opens a long-lived SSE channel for unsolicited server-to-client traffic. Optional: a server may return 405, in which case server messages only arrive on POST streams.
• DELETE on close, with the captured Mcp-Session-Id.
• MCP-Protocol-Version header echoed on every request after the initialize response has been processed by the client.
• 401 with WWW-Authenticate triggers the configured auth refresh; the original request is retried once.

Each parsed SSE event's data: payload is forwarded to the owning client as {mcp_in, self(), Json}. The owner sees the same shape as it does from the stdio transport.