Fix regeneration of freshly generated chat messages

[julien-nc]

Short version

This fixes regeneration of messages that were freshly generated. The task ID is now stored in the message entry so we can get a message from a task ID. That is helpful for the /chat/check_generation?taskId=123 endpoint.

Long version

When a chat task has finished, a listener stores a message in the oc_assistant_chat_msgs table.
In the UI, when a message is currently being generated, the /chat/check_generation?taskId=123 assistant endpoint is polled.
When the related controller sees that the task has finished and is successful:

• it used to generate and return a fake message object on the fly, based on the data in the finished task (because it could not get the message in the DB based on the task ID)
• it now gets the real message that was stored by the listener

The problem was that the message obtained via /chat/check_generation?taskId=123 had no ID so it could not be regenerated because we do it with a request like /chat/regenerate?messageId=619&sessionId=48 but we didn't have an ID for the freshly generated messages.

The issue was not so visible because when opening the assistant, the message list was correctly loaded (from DB entries), with all message having an ID.

[kyteinsky]

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