feat: implement Preference for AI Chat Persisted Session Limit

[CamilleLetavernier]

fixes #16560

What it does

• add a preference AI Features > Chat > Persisted Session Limit with values:
  • -1: unlimited persisted sessions
  • 0: no persisted sessions
  • n>0: n persisted sessions
• update the persisted sessions mechanism to take the preference into account and support the new 0 and -1 cases

How to test

• test all 3 values (-1, 0, and n>0) and make sure the behavior matches the expectations above
  • note: persisted sessions are the sessions saved to disk. They are different from in-memory sessions. Refresh your browser tab when you want to check what is currently persisted, as in-memory sessions have no limit
  • note: sessions are trimmed only when a new session is available. Changing the setting doesn't have an immediate effect; you need to start a new conversation in order to update the current persisted sessions

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines
• User-facing text is internationalized using the nls service (for details, please see the Internationalization/Localization section in the Coding Guidelines)

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[ndoschek]

Thanks for this fix @CamilleLetavernier, works great for me! 👍
Just a small question inline if you have a minute, TIA!

[ndoschek]

Thanks! LGTM 👍