Previously the agent proceeded to the main loop without confirming
the server accepted the channel join. A rejected join (e.g. invalid
token) left the agent running but silently non-functional. Now waits
for the join reply with a 10s timeout, returns error on rejection.
Reader goroutine starts before join send so the reply can be received.
Writer goroutine starts after join confirmation.
Previously, resetting slice length to 0 left previous protobuf data
(device IDs, OID values, credentials) in the backing array. Now zeros
the full capacity before returning to the pool.
handleMessage returns bool to signal session end instead of calling
osExit directly. Defers (pool stop, writeCh close, writerWg wait)
now run before exit, preventing in-flight result loss and panics
from writes to closed channels.
Fix 1: Limit inbound job payload size to 4MB to prevent OOM from malicious server
Fix 2: Worker pool submit accepts context and returns bool, prevents blocking event loop
Fix 3: Panic recovery in worker pool goroutines prevents permanent worker death
Fix 4: WebSocket handshake timeout (30s) prevents indefinite blocking on slow server
Fix 5: Writer goroutine propagates errors to main loop via writeErrCh
Replace unbounded goroutine spawning with fixed-size worker pools:
100 workers for SNMP, 20 for MikroTik, 50 for ping. Prevents file
descriptor exhaustion and device overload when handling thousands
of concurrent jobs. Pool provides backpressure when saturated.
