Streamable HTTP client transport (Req-based).

{Noizu.MCP.Client,
 transport: {Noizu.MCP.Transport.StreamableHTTP.Client, url: "https://example.com/mcp"}}

Every outbound message is an HTTP POST; JSON responses and SSE streams are both handled, the Mcp-Session-Id is captured from the initialize response and echoed on subsequent requests along with MCP-Protocol-Version, and a long-lived GET stream (with Last-Event-ID reconnect/resume) carries unsolicited server→client traffic.

Options: :url (required), :headers (extra request headers), :req_options (merged into every Req call).