edits: interim nicer play with background sessions (#280224)

* edits: interim nicer play with background sessions

- Keep a chat model alive (when enabled) as long as edits are pending
- Eagerly revive chat sessions that have pending edits on boot
- Disable the "Keep/Undo" dialog when closing the editor if the session
  would be kept alive.

Ref #278654 / #277318

* fix tests

* Apply suggestions from code review

Co-authored-by: Copilot <[email protected]>

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Co-authored-by: Copilot <[email protected]>