Fix modal focus trapping for elements excluded from tab order

[huytdps13400]

When a modal contains a ScrollView with focusable={false} or tabIndex={-1}, reverse keyboard navigation can get stuck on the scroll container. The trap calls .focus() on candidates, which succeeds even for elements explicitly excluded from the tab order. Its first/last focus comparison therefore never reaches the buttons inside the scroll view.

Skip candidates with an explicit negative tabindex when choosing a focus-trap destination, while continuing to search their descendants. This also avoids selecting excluded buttons at the edges of a modal. Explicit programmatic focus inside the modal remains allowed, and the existing trap fallback still handles content with no eligible descendants.

Fixes #2823 for the reported focusable={false} / tabIndex={-1} configuration.

Validation

• Added five regression/control tests. On the base, the Modal suite has 4 failures and 33 passes; after the change, all 37 tests pass.
• Chrome 153, React 18.3.1, rendered Modal + overflowing ScrollView: four consecutive Shift+Tab presses previously focused the excluded scroll container every time. After the change, both Tab and Shift+Tab alternate between the first and last buttons for both supported prop configurations.
• Browser controls cover contenteditable without an explicit tabindex, fallback when all descendants are excluded, and restoring focus to the trigger after Escape closes each modal.
• Full DOM and Node suites: 726 tests passed, 6 skipped, 220 snapshots passed. Full lint, formatting and library build pass.
• Flow 0.148 cannot run on this ARM host: its bundled macOS binary is x86_64 and exits with spawn error −86. Flow checking is unverified locally.

The browser can also programmatically focus a scroll container with no explicit tabindex. That existing behavior remains unchanged; this patch specifically honors elements explicitly excluded from the tab order.

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