Feishu WebSocket: No exponential backoff on reconnect

## Bug

Feishu WS returns `system busy` (1000040345). OpenClaw reconnects at fixed ~2min with no backoff — prolonging outage.

Log error: `Cannot read properties of undefined (reading PingInterval)`

## Expected
1. Exponential backoff (1→2→4min cap)
2. Fix race condition on closed WS
3. Optional: health-check before reconnect

## Env
- Node v22 / macOS / 3 Feishu accounts