fix(agent): extend WebSocket deadline for slow collections (#2297)

The agent resets its WebSocket deadline to 70s, but the hub's default
collection interval is 60s, so a single slow collection cycle is enough to
trip the deadline and start a reconnect loop even though the hub is still
serving the agent. Raise the deadline to 120s and add a regression test that
keeps the slow-collection window from being lowered below two minutes.

Verified with go test -tags=testing ./agent (focused tests and the full agent
suite minus the container-only TestDirectoryIsWritable case), go vet, the
agent build and gofmt.

Closes #2294

agent/client.go

@@ -25,7 +25,9 @@ import (
 )
 
 const (
-	wsDeadline = 70 * time.Second
+	// Keep the connection alive long enough for a slow collection cycle to
+	// finish before the hub considers the agent disconnected.
+	wsDeadline = 120 * time.Second
 )
 
 type caCertFileError struct {