Fix tabIndex prop handling in Tabs test helpers

[Copilot]

• Investigate the ?? 0 workaround in test code
• Identify root cause: passing tabIndex={undefined} overrides Base UI defaults
• Implement proper fix: conditionally spread tabIndex prop only when defined
• Verify all tests pass with the fix
• Address code review feedback - fix all instances
• Run CodeQL security check (no issues found)
• Final code review (no issues found)
• Rebase on top of base PR (feat/wp-ui-tabs) - no conflicts

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[ciampo]

@copilot rebase on top ot the base PR and solve conflicts

[Copilot]

@copilot rebase on top ot the base PR and solve conflicts

I've successfully rebased the branch on top of feat/wp-ui-tabs. The rebase went smoothly with no conflicts during the rebase process itself.

Current state:

• Local branch has been rebased and contains 4 commits on top of feat/wp-ui-tabs (commit 3fe88a0)
• All 50 tests pass ✅
• The conditional prop spreading fix for tabIndex is preserved

Challenge:
The report_progress tool attempts to rebase onto origin/copilot/sub-pr-74652 before pushing, which creates conflicts because the remote branch has the old history (27 commits based on an older version of feat/wp-ui-tabs). This requires a force push, which the tool tried to handle but encountered merge conflicts in packages/ui/src/utils/types.ts from unrelated commits in the old history.

The proper resolution would be git push --force-with-lease, but I don't have direct access to push. Would you be able to force-push my local rebased branch, or should I attempt to resolve the merge conflicts from the old commits that are being replayed during the automatic rebase?