Keep hidden channels out of the channel list when member or channel updates arrive - #6702

Goal

A channel hidden via channel.hidden is put back into the channel list by a channel.updated or
member.updated for the same channel, and stays there until the app restarts. Neither add path checks whether the
channel is hidden.

Port of #6697 to develop.

Part of AND-1530

Implementation

• Skip the add in DefaultChatEventHandler.addIfMembershipUpdated when the cached channel is hidden. This is the
  member.updated path, and the only one that applies to plain channel lists, since the base handler already skips
  channel.updated.
• Skip it in GroupAwareChatEventHandler.routeByGroup when the cached channel is hidden, and remove the channel if
  it is still listed. This is the channel.updated path; the default group resolver always resolves a channel into
  the all sentinel group, so an all grouped query took the add unconditionally.
• ChannelStateLegacyImpl.toChannel() resolves the current user from the userFlow it already holds instead of the
  ChatClient singleton. Same value, and it lets the state layer be exercised without a built client.
  ChannelStateImpl needs no equivalent change, it does not sync unread counts there.

hidden is per-user and absent from channel.updated payloads, so both guards read the cached channel. Per-channel
event handling runs before query handling in the same batch, so for a channel whose state is active in memory the flag
is current. parseChatEventResults falls back to the database for channels that are not active, and the database is
only written after query handling, so on that fallback the guard sees the pre-batch value and does not fire.

The new-message add paths stay unguarded: a non-shadowed message clears hidden in ChannelEventHandler, which is
what makes a hidden channel resurface legitimately. iOS does the same in ChannelVisibilityEventMiddleware, since the
backend sends no channel.visible in that case.

Testing

• EventHandlerSequentialHiddenChannelTest drives EventHandlerSequential over the real StateRegistry and
  LogicRegistry. It replays the reported batch (notification.mark_read, channel.hidden, channel.updated,
  member.updated) against a grouped query, and covers a plain query hidden in one batch and updated in the next.
  Both fail without the guards. A third case asserts the same batch without the hidden event still adds the channel,
  and a fourth asserts a new message brings a hidden channel back.
• Unit coverage for each guard in DefaultChatEventHandlerTest and GroupAwareChatEventHandlerTest, including a
  hidden channel already in the list being removed.
• The behaviour was verified on a device on the v6 branch, comparing builds that differ only by the fix: without it a
  hidden channel returns to the list on the following member.updated, with it the channel stays out and a new
  message still brings it back. This port carries no behavioural difference from that build.

Summary by CodeRabbit

• Bug Fixes
  • Hidden channels are no longer added to channel queries through membership updates.
  • Hidden channels are removed or excluded from grouped channel lists until they become visible again.
  • New messages can correctly bring hidden channels back into standard queries.
  • Unread counts now remain accurate when no user is connected.