Add the DRPC-over-WebSocket endpoint that standalone AI Gateway replicas
connect to. The handler validates the requested API version, upgrades to
a WebSocket, sets up a yamux session, and serves the Recorder,
MCPConfigurator, and Authorizer services, mirroring the embedded
CreateInMemoryAIBridgeServer and the external provisioner daemon serve
transport. While a session is active it records last_used_at for the
authenticating gateway key so operators can safely rotate keys.

The route is registered outside the /aibridge catch-all so its overload
middleware does not apply, and is gated by FeatureAIBridge plus gateway
key authentication. A shared aibridgedserver.Register helper now backs
both the embedded and standalone service registration.

Part of AIGOV-308.

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