[Feature]: MessageBar - Add severe-warning intent

[spenoff]

Area

React Components (@fluentui/react-components)

Describe the feature that you would like added

The v9 MessageBar only supports four intents: success, error, warning, and info. The v8 MessageBar additionally supported severeWarning and blocked variants, which had distinct visual treatments (darker warning color, different icon).

Use case: In account.microsoft.com (AMC), the severeWarning variant is used for the unified storage quota warning - a critical user-facing notification that needs visual distinction from a standard warning. Mapping it to warning loses the severity signal, which impacts user comprehension and action urgency.

Proposed solution: Add a severe-warning intent to the MessageBar component with:

• A visually distinct treatment from warning (darker/more urgent colors)
• A distinct icon (e.g., ShieldError or Warning with filled variant)
• Semantic differentiation in the design system

Additional context

our backup plan is to map severeWarning → warning as a workaround, losing visual severity distinction

Have you discussed this feature with our team

No response

Validations

• Check that there isn't already an issue that requests the same feature to avoid creating a duplicate.

Priority

Medium