feat(fuselage): improve accessibility of Message components

[Mubashir2611]

Proposed changes

This PR enhances accessibility and responsiveness across all Message components to ensure WCAG 2.1 compliance and improve user experience across all devices and assistive technologies.

Accessibility Enhancements:

• Added semantic ARIA roles (role="article", role="group", role="region") to Message, MessageHeader, MessageBody, MessageReactions, and MessageMetrics
• Added descriptive aria-label attributes for screen reader context
• Added aria-busy and aria-selected state indicators to Message component
• Implemented full keyboard navigation support (Enter/Space keys) for MessageReaction and MessageReactionAction
• Changed MessageTimestamp from <span> to semantic <time> element with dateTime prop
• Added aria-pressed state to MessageReaction for toggle indication

Responsive Design Improvements:

• Implemented fluid padding using clamp(8px, 3vw, 20px) for adaptive spacing
• Added breakpoint-based layouts (mobile: column, tablet/desktop: row)
• Added responsive typography with clamp(0.875rem, 2.5vw, 1rem) and proper line-height

Testing:

• Enhanced Message.spec.tsx with accessibility and keyboard navigation tests
• Created comprehensive test suites for MessageBody, MessageHeader, MessageTimestamp, and MessageReaction components
• All tests include jest-axe accessibility validation

Issue(s)

Closes #1819

Further comments

All changes are backward compatible and follow WCAG 2.1 Level AA guidelines. The implementation uses existing theme variables and breakpoint mixins to maintain consistency with the design system.

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