Fix tabIndex prop handling in Tabs test helpers

[Copilot]

The test helpers were using tabIndex={tabObj.tabpanel?.tabIndex ?? 0} as a workaround for failing tests. This was misdiagnosed as a JSDOM bug, but the actual issue was that explicitly passing tabIndex={undefined} overrides Base UI's internal default (tabIndex: hidden ? -1 : 0).

Changes

• Replace ?? 0 workaround with conditional prop spreading in UncontrolledTabs and ControlledTabs test helpers
• Only pass tabIndex prop when explicitly defined in test data, allowing Base UI's defaults to apply

// Before: Forces tabIndex={0}, overriding Base UI behavior
<Tabs.Panel tabIndex={tabObj.tabpanel?.tabIndex ?? 0} />

// After: Omits prop when undefined, preserving Base UI defaults
<Tabs.Panel
  {...(tabObj.tabpanel?.tabIndex !== undefined && {
    tabIndex: tabObj.tabpanel.tabIndex,
  })}
/>

This allows Base UI to manage focusability by default while still supporting explicit overrides (e.g., tabIndex: -1 for non-tabbable panels).

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[Copilot]

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