Closes #32217

Follow up of https://github.com/zed-industries/zed/pull/32301, sorry
about the messy rebase in the previous PR.

Release Notes: 
- Fixed `max_tabs` setting not applying immediately when changed
 
TODO: 
- [x] Fix the off-by-one bug (currently closing one more tab than the
max_tabs setting) while perserving "+1 Tab Allowance" feature.
- [x] Investigate Double Invocation of `settings_changed`
- [x] Write test that:
  - Sets max_tabs to `n`
  - Opens `n` buffers
  - Changes max_tabs to `n-1`
  - Asserts we have exactly `n-1` buffers remaining

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Co-authored-by: Joseph T. Lyons <JosephTLyons@gmail.com>