Adds the concrete transport behind the RelayTransport Protocol — the missing
'later-phase work' the relay scaffold deferred. The gateway dials OUT to the
connector over a WebSocket and speaks the newline-delimited JSON frame protocol
(docs/relay-connector-contract.md; connector src/relay/protocol.ts):
- connect(): opens the ws, sends hello{platform,botId}, starts a background
read loop, and resolves handshake() when the connector's descriptor frame
arrives.
- inbound frames -> the registered InboundHandler (rebuilt into a MessageEvent
via _event_from_wire, mapping the snake_case SessionSource wire form back
onto the gateway dataclasses).
- send_outbound / send_follow_up / get_chat_info: request/response correlated
by a uuid requestId against a per-request future, with a timeout so a caller
never hangs; send_interrupt is fire-and-forget.
- disconnect(): cancels the reader, closes the ws, and fails any in-flight
outbound waiters with a structured error.
RelayAdapter.connect() now negotiates the real CapabilityDescriptor from the
transport and adopts it (_apply_descriptor updates MAX_MESSAGE_LENGTH +
markdown surface), replacing the construction-time placeholder. Lazy
'import websockets' mirrors gateway/platforms/feishu.py; WEBSOCKETS_AVAILABLE
gates construction.
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