Sets the Telegram bot's short description (the line under its name) to
"Online" on gateway connect and "Offline" on clean disconnect, gated
behind extra.status_indicator (off by default).
Telegram bots have no presence/online dot — that's a user-account
feature the Bot API doesn't expose for bots. The short description is
the closest available surface, so this gives users a way to tell whether
the gateway is up from the bot's profile.
- New extra.status_indicator flag (+ status_online/status_offline text
overrides), read in __init__ via config.extra — no config-schema change.
- _set_status_indicator() helper: best-effort, swallows API errors so it
never blocks connect/disconnect; truncates to Telegram's 120-char cap.
- Wired Online after _mark_connected(), Offline at top of disconnect()
while the bot HTTP client is still alive.
- 9 unit tests + Telegram docs section.
Requested by @ilTrumpista, cc @Teknium.
26e76a75e5