Summary

Fixes two issues with WhatsApp platform:

1. Bridge Process Leak (PID file mechanism)

Problem: When the gateway restarts or crashes, old Node.js bridge processes become orphaned. The existing _kill_port_process() relies on fuser (Linux) which is often not available (e.g., WSL2, minimal installations), causing silent failures and process accumulation (up to 39 zombie processes observed).

Solution:

• Added PID file mechanism (bridge.pid in session directory) to track bridge processes
• _kill_stale_bridge_by_pidfile() kills orphaned bridges by PID before spawning new ones
• _write_bridge_pidfile() records the bridge PID after spawning
• PID file is cleaned up on graceful disconnect
• Added lsof as fallback in _kill_port_process() when fuser is not available

2. Config Asymmetry (disable WhatsApp via config)

Problem: WhatsApp can only be enabled via WHATSAPP_ENABLED=true env var, but there's no way to disable it via CLI/UI. Setting whatsapp.enabled: false in config.yaml was ignored.

Solution:

• Check config.yaml for WhatsApp settings first
• Respect explicit disable: WHATSAPP_ENABLED=false env var or whatsapp.enabled: false in config.yaml
• Only fall back to env var for enable if no YAML config exists

Testing

• All WhatsApp tests pass (74 passed, 3 pre-existing flaky failures unrelated to changes)
• Config tests pass
• Syntax verified

Fixes #19124