feat(react-message-bar): add base state hooks for MessageBar components

[Copilot]

Introduces base state hooks for all MessageBar components following the pattern established in #35697 for Persona.

Changes

• MessageBar: Added useMessageBarBase_unstable that handles core behavior (layout, politeness announcements, refs, a11y) without design props (intent, shape, icon)
• MessageBarActions: Added useMessageBarActionsBase_unstable
• MessageBarBody: Added useMessageBarBodyBase_unstable
• MessageBarGroup: Added useMessageBarGroupBase_unstable
• MessageBarTitle: Added useMessageBarTitleBase_unstable

Each component now exports corresponding BaseProps and BaseState types.

Usage

// Create a custom MessageBar variant without design props
const state = useMessageBarBase_unstable(baseProps, ref);

// Full hook composes base + adds design props
const state = useMessageBar_unstable(props, ref);
// Internally: useMessageBarBase_unstable({ ...restProps, politeness }, ref)

API Surface

New exports per component:

• useMessageBar*Base_unstable hooks
• MessageBar*BaseProps types
• MessageBar*BaseState types

---

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[github-actions]

Pull request demo site: URL

[github-actions]

🕵🏾‍♀️ visual changes to review in the Visual Change Report

vr-tests-react-components/CalendarCompat 4 screenshots

Image Name	Diff(in Pixels)	Image Type
vr-tests-react-components/CalendarCompat.multiDayView - Dark Mode.default.chromium.png	555	Changed
vr-tests-react-components/CalendarCompat.multiDayView.default.chromium_1.png	481	Changed
vr-tests-react-components/CalendarCompat.multiDayView.default.chromium.png	391	Changed
vr-tests-react-components/CalendarCompat.multiDayView - High Contrast.default.chromium.png	680	Changed

vr-tests-react-components/Charts-DonutChart 2 screenshots

Image Name	Diff(in Pixels)	Image Type
vr-tests-react-components/Charts-DonutChart.Dynamic - RTL.default.chromium.png	5570	Changed
vr-tests-react-components/Charts-DonutChart.Dynamic - Dark Mode.default.chromium.png	7530	Changed

vr-tests-react-components/Positioning 2 screenshots

Image Name	Diff(in Pixels)	Image Type
vr-tests-react-components/Positioning.Positioning end.chromium.png	610	Changed
vr-tests-react-components/Positioning.Positioning end.updated 2 times.chromium.png	607	Changed

vr-tests-react-components/Skeleton converged 1 screenshots

Image Name	Diff(in Pixels)	Image Type
vr-tests-react-components/Skeleton converged.Opaque Skeleton with circle - High Contrast.default.chromium.png	1	Changed

There were 1 duplicate changes discarded. Check the build logs for more information.

[github-actions]

📊 Bundle size report

Package & Exports	Baseline (minified/GZIP)	PR	Change
react-components react-components: entire library	1.291 MB 322.995 kB	1.292 MB 323.081 kB	155 B 86 B
react-message-bar MessageBar (all components)	23.264 kB 8.58 kB	19.956 kB 7.31 kB	-3.308 kB -1.27 kB

Unchanged fixtures

Package & Exports	Size (minified/GZIP)
react-components react-components: Button, FluentProvider & webLightTheme	70.224 kB 19.89 kB
react-components react-components: Accordion, Button, FluentProvider, Image, Menu, Popover	237.179 kB 68.482 kB
react-components react-components: FluentProvider & webLightTheme	43.523 kB 13.978 kB
react-portal-compat PortalCompatProvider	8.386 kB 2.624 kB
react-timepicker-compat TimePicker	108.15 kB 35.693 kB

_{🤖 This report was generated against 72fa991316ec4a9d207a2926f4bf41c683314bad}