Keep the channel state of a deleted channel and record its deletion date

[gpunto]

Goal

ChannelState.channelData.deletedAt is never populated, so a screen has no way to tell that its channel
was deleted. On the current channel state implementation the deletion also blanks the open screen: the
message list empties and the channel data resets to a placeholder with no name.

Port of #6693 to develop.

Part of AND-1522

Implementation

• Forward a batch to the channel logic before discarding the channel. EventHandlerSequential evicted the
  channel from both registries first, so channel.deleted never reached the channel event handler and the
  branch setting the deletion date was unreachable. Both registries are now evicted right after the batch is
  handled.
• Stop destroying the state of a deleted channel in StateRegistry. It is still dropped from the registry,
  so later lookups build a fresh state, but consumers holding it keep the channel as it was.
  StateRegistry.clear() still destroys everything on logout.
• Handle notification.channel_deleted in both channel event handlers, which had no branch for it.
• Record the deletion date even when the channel data has not loaded yet. ChannelStateImpl.deleteChannel
  copied onto a null value and dropped the deletion, unlike the legacy implementation.
• Stop clearing the messages on deletion. Truncation remains the one event that empties a channel, matching
  iOS, where message visibility is gated on the channel's truncatedAt and never on its deletion date.

Testing

• New EventHandlerSequentialChannelDeletedTest covers both deletion events against both channel state
  implementations, over the real StateRegistry and LogicRegistry, asserting the deletion date reaches an
  active channel's state, its messages survive, and the channel is discarded from the registry.
• Verified on a device: with a channel open, deleting it kept the name and all messages and moved deletedAt
  from null to the deletion date. Without the change, the same run empties the message list and blanks the
  name, with no deletion date.

[github-actions]

PR checklist ✅

All required conditions are satisfied:

• Title length is OK (or ignored by label).
• At least one pr: label exists.
• Sections ### Goal, ### Implementation, and ### Testing are filled, or the PR is bot-authored.
• An issue is linked (Linear ticket or GitHub issue), or the PR is bot-authored.

🎉 Great job! This PR is ready for review.

[github-actions]

SDK Size Comparison 📏

SDK	Before	After	Difference	Status
stream-chat-android-client	6.07 MB	6.08 MB	0.00 MB	🟢
stream-chat-android-ui-components	11.39 MB	11.39 MB	0.00 MB	🟢
stream-chat-android-compose	13.06 MB	13.06 MB	0.00 MB	🟢

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
94.3% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud