fix: v8 ComboBox needs to update aria-activedescendant value

[smhigley]

Previous Behavior

This is a regression introduced in #26574, and causes the aria-activedescendant value to not update when arrowing through options, which makes the control fully broken for screen reader users.

The root cause is that the currentPendingValueValidIndex needs to take precedence as the index of the active option, but the PR updated the logic to use selectedIndices[0] as the primary value, while it should be the fallback value.

New Behavior

aria-activedescendant is now calculated off of currentPendingValueValidIndex, falling back to selectedIndices[0]. This is what the behavior was prior to the PR.

[fabricteam]

📊 Bundle size report

_{🤖 This report was generated against 1071a2a382741b1328228b336c2a94de388a3963}

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

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Latest deployment of this branch, based on commit c19d115:

Sandbox	Source
@fluentui/react 8 starter	Configuration
@fluentui/react-components 9 starter	Configuration

[size-auditor]

Asset size changes

Size Auditor did not detect a change in bundle size for any component!

Baseline commit: af3c827937dcb0a32f52ef8a93b91c1276956b56 (build)

[fabricteam]

Perf Analysis (@fluentui/react)

No significant results to display.

All results

Scenario	Render type	Master Ticks	PR Ticks	Iterations	Status
BaseButton	mount	818	827	5000
Breadcrumb	mount	2180	2227	1000
Checkbox	mount	2157	2109	5000
CheckboxBase	mount	1959	1906	5000
ChoiceGroup	mount	3757	3713	5000
ComboBox	mount	884	900	1000
CommandBar	mount	8035	7990	1000
ContextualMenu	mount	17450	17292	1000
DefaultButton	mount	967	984	5000
DetailsRow	mount	2901	2905	5000
DetailsRowFast	mount	2855	2882	5000
DetailsRowNoStyles	mount	2648	2607	5000
Dialog	mount	3440	3545	1000
DocumentCardTitle	mount	329	325	1000
Dropdown	mount	2575	2534	5000
FocusTrapZone	mount	1484	1474	5000
FocusZone	mount	1445	1426	5000
GroupedList	mount	50209	58642	2
GroupedList	virtual-rerender	24363	24311	2
GroupedList	virtual-rerender-with-unmount	75406	74630	2
GroupedListV2	mount	317	312	2
GroupedListV2	virtual-rerender	296	312	2
GroupedListV2	virtual-rerender-with-unmount	315	324	2
IconButton	mount	1410	1399	5000
Label	mount	444	456	5000
Layer	mount	3570	3617	5000
Link	mount	534	525	5000
MenuButton	mount	1234	1217	5000
MessageBar	mount	27294	27325	5000
Nav	mount	2528	2561	1000
OverflowSet	mount	1022	1019	5000
Panel	mount	2264	2211	1000
Persona	mount	968	980	1000
Pivot	mount	1184	1175	1000
PrimaryButton	mount	1103	1097	5000
Rating	mount	5833	5748	5000
SearchBox	mount	1133	1149	5000
Shimmer	mount	2302	2347	5000
Slider	mount	1713	1718	5000
SpinButton	mount	3743	3799	5000
Spinner	mount	528	525	5000
SplitButton	mount	2430	2435	5000
Stack	mount	536	530	5000
StackWithIntrinsicChildren	mount	1129	1159	5000
StackWithTextChildren	mount	3214	3205	5000
SwatchColorPicker	mount	8132	8057	5000
TagPicker	mount	2014	2006	5000
Text	mount	498	503	5000
TextField	mount	1193	1199	5000
ThemeProvider	mount	1189	1134	5000
ThemeProvider	virtual-rerender	772	761	5000
ThemeProvider	virtual-rerender-with-unmount	1749	1729	5000
Toggle	mount	795	811	5000
buttonNative	mount	289	284	5000