fix(Tree): Roving tabindexes should be refreshed if current element is unmounted

[ling1726]

Follow up from #31595

The current tree item with tabindex 0 can be unmounted but kept as a reference in side the navigation state. This causes the update to be skipped even when in reality there is no tabbable item.

Adds an extra check that the current item is in DOM to fix for this case and improves the test to handle this case

[fabricteam]

Perf Analysis (@fluentui/react-components)

Scenario	Render type	Master Ticks	PR Ticks	Iterations	Status
FluentProviderWithTheme	mount	78	81	10	Possible regression
FluentProviderWithTheme	virtual-rerender	31	39	10	Possible regression

All results

Scenario	Render type	Master Ticks	PR Ticks	Iterations	Status
Avatar	mount	636	658	5000
Button	mount	290	306	5000
Field	mount	1193	1132	5000
FluentProvider	mount	727	710	5000
FluentProviderWithTheme	mount	78	81	10	Possible regression
FluentProviderWithTheme	virtual-rerender	31	39	10	Possible regression
FluentProviderWithTheme	virtual-rerender-with-unmount	84	76	10
MakeStyles	mount	870	867	50000
Persona	mount	1744	1704	5000
SpinButton	mount	1412	1369	5000
SwatchPicker	mount	1666	1729	5000

[fabricteam]

📊 Bundle size report

Package & Exports	Baseline (minified/GZIP)	PR	Change
react-components react-components: entire library	1.095 MB 270.816 kB	1.095 MB 270.834 kB	70 B 18 B

Unchanged fixtures

Package & Exports	Size (minified/GZIP)
react-components react-components: Button, FluentProvider & webLightTheme	69.141 kB 20.157 kB
react-components react-components: Accordion, Button, FluentProvider, Image, Menu, Popover	211.741 kB 60.957 kB
react-components react-components: FluentProvider & webLightTheme	44.442 kB 14.607 kB
react-portal-compat PortalCompatProvider	8.39 kB 2.64 kB
react-timepicker-compat TimePicker	106.559 kB 35.517 kB

_{🤖 This report was generated against 385ad908f11f5669580b9b1fec6e5e76be30f770}