fix(a11y): keyboard close and a 24 px close target in QueryaTabStrip

[ZhuchkaTriplesix]

Closes #1372

Problem

• The close icon was a 13 px glyph with 1 px padding: a ~15 px pointer target.
• A focused tab ignored Delete / Backspace, so a keyboard user could not close it.
• The close button had no semantics of its own: the whole tab was inside ExcludeSemantics.

Change

• _onKeyEvent: Delete and Backspace call onClose(index), only when canClose allows it.
• The close hit area is a 24x24 box. The tab's padding around it shrinks (left 10, right 5, vertical 2 dense / 6 standard) so tabs keep their width and height.
• ExcludeSemantics now wraps only the label text. The close button is a container Semantics(button, label: 'Close <tab> tab') inside a tooltip, so it is a separate node under the tab.

Not in this PR

The close button is not a separate Tab stop; Delete / Backspace on the focused tab is the keyboard path. Golden querya_tab_strip.png (tag golden, excluded from CI) may need a refresh if the 1 px padding shift is visible.

Tests

test/shared/querya_tab_strip_test.dart: Delete, Backspace, canClose respected, and the close button's size, semantics label, tooltip and tap.

Not run locally; CI is the check.