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

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

vr-tests-react-components/Charts-DonutChart 3 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/Charts-DonutChart.Dynamic.default.chromium.png	5581	Changed

vr-tests-react-components/Positioning 2 screenshots

Image Name	Diff(in Pixels)	Image Type
vr-tests-react-components/Positioning.Positioning end.updated 2 times.chromium.png	26	Changed
vr-tests-react-components/Positioning.Positioning end.chromium.png	732	Changed

vr-tests-react-components/TagPicker 4 screenshots

Image Name	Diff(in Pixels)	Image Type
vr-tests-react-components/TagPicker.disabled - Dark Mode.disabled input hover.chromium.png	658	Changed
vr-tests-react-components/TagPicker.disabled - RTL.disabled input hover.chromium.png	635	Changed
vr-tests-react-components/TagPicker.disabled - High Contrast.chromium.png	1319	Changed
vr-tests-react-components/TagPicker.disabled.chromium.png	677	Changed

There were 4 duplicate changes discarded. Check the build logs for more information.