Slack/CSV Import: Messages do not appear after successful import

[CodeMatrix1]

Description

During Slack or CSV import, channels and users are created successfully, but imported messages do not appear in the corresponding channel timelines after the import completes.

The import process finishes without fatal errors, however message conversion logs repeatedly show importer-message-unknown-room errors. In some scenarios where rooms are already there, messages are inserted into the database but remain invisible in the channel UI.

This results in imported workspaces appearing empty despite successful import completion.

Steps to reproduce

1. Go to Administation>Import
2. Perform a Slack or CSV import with a zip containing channels and messages.
3. Allow the import process to complete.
4. Open one of the imported channels.

Expected behavior

Imported messages should appear normally in the channel timeline after import, matching the source Slack/CSV data.

Actual behavior

• Channels are created successfully.
• Channels appear in the room directory.
• Import progress completes without failure.
• But Messages are missing from the channel timeline.

Resolution Needed

1. Newly created rooms are registered in the importer cache before message conversion begins.
2. Optional message metadata fields are omitted when not explicitly defined to preserve normal message rendering behavior.

Video

In the imported slack data, I had messages in channels 'social' and 'all-mine' but they do not appear on root.

6818727f-b35e-42d6-80ab-50d968c895b6.mp4