TreeDataGrid: focus the parent row on ArrowLeft

[nstepien]

Pressing ArrowLeft on a collapsed child group row makes its parent group row active, but focus stays on the child row. Other keyboard navigation in DataGrid and clicking a group row both pass { shouldFocus: true }, so this path now does too.

The existing "cell navigation in a treegrid" test already says the parent group should be focused, but it only checked the active class. It now also asserts toHaveFocus(); that assertion fails without the fix.

Also fixes two comments in the row selection handler that described the select and unselect branches the wrong way round.

Verified locally: typecheck, lint, format:check, lint:md, and npm test (529 passed).

🤖 Generated with Claude Code

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.30%. Comparing base (b786d20) to head (a279682).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4202   +/-   ##
=======================================
  Coverage   98.30%   98.30%           
=======================================
  Files          45       45           
  Lines        1943     1943           
  Branches      732      732           
=======================================
  Hits         1910     1910           
  Misses         33       33           

Files with missing lines	Coverage Δ
src/TreeDataGrid.tsx	94.41% <100.00%> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.