fix(grid): use `auto` tooltip positioning for `Pane.SplitterButton`

[jzempel]

Description

This PR addresses an issue where default tooltip positioning can result in cutting off the tooltip when a pane is expanded/contracted to the edge of the visible screen/container.

Checklist

• [ ] :ok_hand: ~design updates will be Garden Designer approved (add the designer as a reviewer)~
• [ ] :globe_with_meridians: ~demo is up-to-date (npm start)~
• [x] :arrow_left: renders as expected with reversed (RTL) direction
• [ ] :black_circle: ~renders as expected in dark mode~
• [ ] :metal: ~renders as expected with Bedrock CSS (?bedrock)~
• [ ] :guardsman: ~includes new unit tests. Maintain existing coverage (always >= 96%)~
• [ ] :wheelchair: ~tested for WCAG 2.1 AA accessibility compliance~
• [x] :memo: tested in Chrome, Firefox, Safari, and Edge

[coveralls]

coverage: 95.659% (-0.2%) from 95.9% when pulling 3052fd7eb49db270a9f18a20729c281fc233c333 on jzempel/splitter-tooltip into bcbb17c01aa95abe1ffabf42eb586997699cbeb8 on main.