supabase / supabase-flutter / 39969092550

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fix(realtime)!: rejoin after disconnect and keep one channel per name (#2030)

Fixes
[SDK-2338](https://linear.app/supabase/issue/SDK-2338/fixrealtime-channel-lifecycle-bugs-around-disconnect-duplicate-names).

Fixes the six channel lifecycle findings from the v3 review. Each one
has a regression test on the fake transport (`MockRealtimeTransport`
from `supabase_test`, or mocktail stubs where the fake cannot model a
socket that never closes). The tests are in
`test/channel_lifecycle_test.dart`.

## Changes

1. **Rejoin after `disconnect()` then connect.** `disconnect()` marks
every subscribed channel as errored, without reporting an error, whether
or not a connection is open. It also cancels the pending join reply and
the rejoin backoff. The next connection rejoins those channels through
the existing errored-channel rejoin. Internal reconnects use a private
`_disconnect()` that leaves the channels alone, so a dropped connection
still reports `channelError` and times out the way it did before.
supabase_flutter no longer needs its `forceRejoin` loop on resume, so it
is removed. `forceRejoin` is merged into `rejoin`. The class dartdoc no
longer claims that `disconnect` removes the channels.
2. **Same channel name.** `channel()` now returns the channel already
registered for that name, as realtime-js does, and ignores the options
passed with the repeated call. A channel that is leaving is skipped, so
a name can be reused right after `unsubscribe()`. `subscribe()` on a
channel that has not closed now returns the channel instead of throwing,
as in realtime-js, so code that subscribes a shared name again keeps
working. Subscribing a closed channel still throws. `leaveOpenTopic` was
only reachable through duplicate registrations, so it is removed.
`unsubscribe()` no longer buffers a `phx_leave` while the socket is
disconnected, because the server has already dropped the channel and a
late leave could reach it after a newer join on the same topic.
3. **Reply b... (continued)

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