Proxy MCP Streamable HTTP to the pooled query fleet.

The response is streamed rather than buffered. MCP replies may be text/event-stream, and reading one to completion before returning would defeat the point of a streaming transport and stall the client until the whole exchange finished.

Three credentials reach this door. An OAuth bearer from the broker takes the armed path (verify, resolve the active workspace, mint a fleet token) or, with no signer configured, the dark path that forwards it unchanged for the query service to verify independently. A gfy_mcp_ Workspace API Token takes the token path instead: it names its own workspace, so it is verified here and a fleet token is minted for it -- the raw token is never forwarded.