Replace per-request aiohttp.ClientSession() in every WhatsApp adapter
method with a single persistent self._http_session, matching the pattern
used by Mattermost, HomeAssistant, and SMS adapters.
Changes:
- Create self._http_session in connect(), close in disconnect()
- All bridge HTTP calls (send, edit, send-media, typing, get_chat_info,
poll_messages) now use the shared session
- Explicitly cancel _poll_task on disconnect() instead of relying
solely on self._running = False
- Health-check sessions in connect() remain ephemeral (persistent
session not yet created at that point)
- Remove per-method ImportError guards for aiohttp (always available
when gateway runs via [messaging] extras)
Salvaged from PR #1851 by Himess. The _poll_task storage was already
on main from PR #3267; this adds the disconnect cancellation and the
persistent session.
Tests: 4 new tests for session close, already-closed skip, poll task
cancellation, and done-task skip.
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