Without this UA tag the Feishu server does not push group @mention events
over the WebSocket transport. The "channel" tag tells the server to use
the Channel protocol which enables group-message routing in addition to P2P
direct messages.
Root cause: FeishuWSClient was created without any UA signaling tag, so the
server defaulted to the basic DM-only push mode. Group @mention events were
silently dropped before reaching Hermes.
Fixes https://github.com/NousResearch/hermes-agent/issues/50656
Also adds a regression test verifying the UA tag is present in the
FeishuWSClient constructor call.
949e4cb72a