fix(autocmd): guard number disable callback with is_disabled()

[snoblenet]

Draft PR for self-review -- NOT READY FOR PUBLIC REVIEW

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The BufLeave/WinLeave callback for config.ui.number unconditionally set vim.wo.number = false, unlike every sibling callback in this file (signcolumn, cursorline, cursorcolumn, colorcolumn, relativenumber, hybridnumber), which all check utils.is_disabled() first. This meant toggling vim.g.focus_disable mid-session still stripped line numbers from the window being left, instead of leaving it untouched like the other options.

Add the same is_disabled() guard used elsewhere, and add a regression test in tests/test_ui.lua modeled on the existing signcolumn disable guard test.

Manual Testing

1. Configure focus.nvim with ui = { number = true }, open a file, and :vsplit.
2. Confirm number is on in the active split (:set number?).
3. Disable focus.nvim mid-session: :let g:focus_disable = v:true.
4. Move focus to the other split (<C-w>w) — this fires WinLeave on the window just left.
5. Check the window just left: number should still be on (unchanged), matching how signcolumn/cursorline/etc. already behave when disabled. Before this fix, number would incorrectly turn off here.
6. Re-enable focus.nvim: :let g:focus_disable = v:false, then move focus between the splits again and confirm number still correctly follows focus (on in the active window, off in the inactive one) — normal behavior is unaffected.
7. Cross-check with the automated regression test: make test_file FILE=tests/test_ui.lua (all cases pass, including the new number respects disabled state on WinLeave).

Checks

• make test — passes (aside from one pre-existing, environment-specific failure in tests/test_autoresize.lua unrelated to this change: sandbox denies spawning /bin/zsh for a terminal test)
• make test_file FILE=tests/test_ui.lua — passes, 15/15
• stylua --check . — passes, no formatting issues
• make lint (selene) — 16 pre-existing warnings (unused variables in tests/test_functions.lua, tests/test_splits.lua, tests/test_ui.lua, tests/test_autoresize.lua, and vendored tests/mini.test/), all predating this branch (blamed to a 2023 commit). No new warnings introduced by this change.