[dialog] Honor explicit final focus on backdrop dismissal - #6248

After a modal dialog closes on a backdrop press in a browser that ignores focus({ preventScroll }), focus can fall back to document.body even when the app's finalFocus callback returns a specific element. In the reported scenario, this sends the screen reader back to the top of the page.

Honor an explicit focus target on this dismissal path. Keep the existing scroll protection when finalFocus resolves to the default behavior. The target the application explicitly supplies may scroll into view on browsers that ignore the option.

Fixes #6161.

To reproduce, render a modal Dialog with a backdrop and set Dialog.Popup finalFocus={() => continueButton.current}. Open from the trigger, then press the backdrop. On Base UI 1.9.0 (master at 315eba7), with React 19.3.1 and the browser behavior simulated by ignoring focus options, the dialog closes and document.activeElement is document.body. The same test passes after this change and the explicitly specified Continue button has focus. The control case where the callback returns true keeps the existing behavior.

Validation on Node 26.11.1:

• New regression passed after the fix; before the fix the explicit case failed and the default control passed.
• Dialog and focus manager suites: 333 passed in jsdom and 388 passed in Chromium (Chrome Headless Shell 158.0.8127.18).
• Workspace type check, repository ESLint and lint rule tests, Stylelint, Prettier, package build, and git diff --check passed.
• The test simulates unavailable focus options. No physical Android device was used.

AI assistance disclosure: Codex helped reproduce the report, implement the patch and regression, and run the checks above.

• I have followed (at least) the PR section of the contributing guide.