Activate next tab (below) after closing the active vertical tab

[elviskahoro]

Summary

Closing the active tab should move focus to the next tab — the tab to the
right (horizontal) or below (vertical) — falling back to the previous neighbor
only when you close the last tab. This is the standard convention set by
browsers and macOS apps.

Today the behavior is inconsistent between layouts:

• Horizontal tabs already do the right thing. PR Activate right horizontal tab after close #11197 ("Activate right
  horizontal tab after close") changed the post-close selection from
  index.saturating_sub(1) (previous tab) to index.min(self.tabs.len() - 1)
  (next tab).
• Vertical tabs still activate the previous (up) tab after a close.

This is the part that confused us: PR #11197 explicitly scoped its fix to
horizontal mode and called out that it would "preserve existing vertical tab
behavior." It did so by gating the new logic behind uses_vertical_tabs(ctx):

let active_index = if uses_vertical_tabs(ctx) {
    index.saturating_sub(1)         // vertical: previous/up tab
} else {
    index.min(self.tabs.len() - 1)  // horizontal: next/right tab
};

We couldn't find a reason vertical tabs should intentionally focus the
previous tab while horizontal focuses the next one — it breaks the same
convention #11197 set out to fix, just in the other layout. If there is a
deliberate reason for the asymmetry, please flag it on this PR.

Change

Drop the layout branch in Workspace::remove_tab and always use the
next-neighbor rule for both layouts:

// Activate the tab that was immediately after the closed one — to the right
// for horizontal tabs, below for vertical tabs. After removal that tab
// occupies the same index. If the closed tab was the last one, fall back to
// the new last tab (the previous neighbor).
let active_index = index.min(self.tabs.len() - 1);
self.activate_tab_internal(active_index, ctx);

remove_tab is the single point all close paths funnel through (keyboard, the
tab "X" button, the vertical-tabs panel, and tab-group closes), so this fixes
every entry point. Horizontal behavior is unchanged; only vertical tabs change
(previous → next).

Tests

• Existing horizontal tests still pass (no regression):
  test_close_active_horizontal_tab_activates_tab_to_right,
  test_close_last_horizontal_tab_activates_tab_to_left.
• Added vertical coverage mirroring them:
  test_close_active_vertical_tab_activates_tab_below (would have failed before
  this change), test_close_last_vertical_tab_activates_tab_above (fallback).
• cargo check -p warp --tests clean; the four *_tab_activates_tab_* tests
  pass.

Closes #13143

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[oz-for-oss]

@elviskahoro

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[oz-for-oss]

@elviskahoro

Every PR must be linked to a same-repo issue before Oz can review it.

Next step: open or find a same-repo issue describing this change, then link it to this PR by adding Closes #123 to the PR description (or using the "Development" sidebar on GitHub). A maintainer will mark the issue ready-to-implement when it is ready. Once it is marked, comment /oz-review to re-trigger review.

See the contribution guidelines for the full readiness model.

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[elviskahoro]

@oz-agent This PR closes #13143 #13143

[elviskahoro]

@bholmesdev I did the CLA check! lmk if this needs feature flag