feat: update queue item on session completion

[psychedelicious]

Summary

feat(app): update queue item's session on session completion

The session is never updated in the queue after it is first enqueued. As a result, the queue detail view in the frontend never never updates and the session itself doesn't show outputs, execution graph, etc.

We need a new method on the queue service to update a queue item's session, then call it before updating the queue item's status.

Queue item status may be updated via a session-type event or queue-type event. Adding the updated session to all these events is a hairy - simpler to just update the session before we do anything that could trigger a queue item status change event:

• Before calling emit_session_complete in the processor (handles session error, completed and cancel events and the corresponding queue events)
• Before calling cancel_queue_item in the processor (handles another way queue items can be canceled, outside the session execution loop)

When serializing the session, both in the new service method and the get_queue_item endpoint, we need to use exclude_none=True to prevent unexpected validation errors.

Related Issues / Discussions

Closes #5933

QA Instructions

• On main, generate an image & wait for it to finish
• Check the queue tab and expand the queue item detail view
• Observe that the execution graph, results and such are empty
• Check out this PR
• Do the same thing
• Observe that the queue item detail view shows the executed session
• Invoke a workflow that will error (e.g. integer primitive of 0 into a resize image node's width)
• Observe that the queue item detail view shows a partially executed session w/ error

Merge Plan

This is based on #5748. I'll merge and update this PR as required after that one is merged.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)

[psychedelicious]

Marked as draft to prevent premature merge

[psychedelicious]

superseded by #6419