[AND-550] Add pending messages support in LLC.

[VelikovPetar]

🎯 Goal

Introduces a way to access the pending messages in the channel for the current user. Additionally, exposes an operation to fetch a pending message (and its metadata) by its ID. Docs: https://getstream.io/chat/docs/react/pending_messages/

🛠 Implementation details

• Add pendingMessages field to Channel: Holds the pending messages for the current user in the given channel.
• Introduce ChatClient.getPendingMessage method to fetch a pending message and its metadata by its IDs. Note: This operation uses the same endpoint as the getMessage operation. I introduced a new method, because it is not possible to return the pending message metadata without a breaking change on the getMessage method. I also wanted to avoid introducing a new filed in the Message object, as the metadata doesn't really belong to it. This would potentially be a discrepancy between different SDKs, but I wasn't able to find a different way to do it without a breaking change.
• Add markMessagesPending flag to the channel Config field - indicator if pending messages are enabled for the channel.

Note: These additions are not used internally, they are just exposed for client integration.

🎨 UI Changes

NA

🧪 Testing

1. Enable pending messages in the App dashboard (or contact me to provide a test user/apikey)
2. Send a message
3. Call queryChannels
4. Observe the response field: result.channels[x].pendingMessage -> It should contain all pending messages for the user in the channel
5. Observe the response field: result.channels[x].config.markMessagesPending -> should be true otherwise false based on the dashboard config

[github-actions[bot]]

SDK Size Comparison 📏

SDK	Before	After	Difference	Status
stream-chat-android-client	3.20 MB	3.21 MB	0.00 MB	🟢
stream-chat-android-offline	3.43 MB	3.43 MB	0.00 MB	🟢
stream-chat-android-ui-components	10.50 MB	10.50 MB	0.00 MB	🟢
stream-chat-android-compose	12.69 MB	12.69 MB	0.00 MB	🟢

[github-actions[bot]]

DB Entities have been updated. Do we need to upgrade DB Version? Modified Entities :

stream-chat-android-offline/src/main/java/io/getstream/chat/android/offline/repository/domain/channelconfig/internal/ChannelConfigEntity.kt

[sonarqubecloud[bot]]

Quality Gate failed

Failed conditions
63.3% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud

[VelikovPetar]

Moved the PR to Draft - there are some newer changes/requirement that need to be done.

[sonarqubecloud[bot]]

Quality Gate failed

Failed conditions
60.9% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud

[VelikovPetar]

Should we keep "pending messages" within our local db as well??

I was considering this, but given the fact that currently the Channel.pendingMessages field is not used in the state/offline layers, and is just exposed on the Channel model from the LLC - I am not sure if it makes too much sense to also store them in the DB, as they will never be accessed. I think it will just introduce overhead if we store the pending messages but never read them (of course, this will need to change if we eventually add specific state/ui handling of pending messages)

CC @JcMinarro