Closes https://github.com/zed-industries/zed/issues/31870

Release Notes:

- Allowed opening 1 more item if `n` tabs are pinned, where `n` equals
`max_tabs` count.
- Fixed a bug where pinned tabs would eventually be closed out when
exceeding the `max_tabs` count.
- Fixed a bug where a tab could be lost when pinning a tab while at the
`max_tabs` count.
- Fixed a bug where pinning a tab when already at the `max_tabs` limit
could cause other tabs to be incorrectly closed.