dock: Keep a split filled when its last slot is hidden (#2788)

## Summary

A dock split pins every slot to a fixed size with `flex_none` and leaves
one flexible, to absorb whatever the container has spare. `render_node`
picked that slot by tree position — the last child — without asking
whether the slot is drawn at all.

A slot whose container holds only hidden panels renders a bare `div` and
grows nothing. When that is the trailing slot, every drawn slot stays
rigid and the split stops short of its frame, leaving a band of
`split_frame`'s own `tab_bar` background under the last visible panel.
Hiding a slot is an everyday event: a panel that only applies to some
symbols answers `visible` with `false` for the rest.

This is a regression from #2772. The old `StackPanel::render` never
sized a slot, so every one of them carried `ResizablePanel`'s
`flex_grow: 1` and the leftover was always shared out; the rigid slots
arrived with the layout engine.

`render_node` now picks the last *shown* slot.

The new test builds a three-slot dock over `HideableProbe`, measures
what each slot is really drawn at, and walks every subset of hidden
slots. Only the subsets that hide the trailing slot regress — a 1080px
dock left 277px empty — which is what makes the fix positional rather
than a special case.

No public API change.

## Test plan

- [x] `cargo test -p gpui-base --lib` — 555 passed
- [x] `cargo test -p gpui-component --lib` — 447 passed
- [x] `cargo clippy -p gpui-base -p gpui-component --all-targets` — no
new warnings
- [x] `cargo fmt --check` — clean for the changed files
- [x] Reverting the one-line pick makes
`a_split_fills_its_container_whichever_slots_are_hidden` fail on exactly
the trailing-slot subsets

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>