Feature: add variant to reveal message actions on hover/focus

[nicolethoen]

Based on the results of the Compass/unified theme dev/design sync on Jan 14 and as a follow up to patternfly/patternfly-design-kit#908

Summary

Implement Opt-in Hover/Focus Visibility for Message Actions

Description

Update the Message component to allow consumers to hide actions until hover/focus. Implemented as an opt-in feature (e.g., via a new prop) and must not change the default behavior of existing chat implementations.

Functional Requirements

• Opt-in Prop: Create a mechanism (e.g., hideActionsUntilHover) that defaults to false.
• Visibility Logic:
  • When enabled, actions should be visually hidden (opacity: 0 or visibility: hidden) by default.
  • Actions must occupy their full layout space at all times to prevent content shifting/jumping when toggled.
  • Trigger visibility on hover of the message container or focus-within of the action group.
• Accessibility (A11y):
  • Keyboard users must be able to Tab into the actions.
  • When an action receives focus, the action group must become visually visible.
  • Ensure the "hover" area is large enough to avoid pointer-precision issues (Message + Action container).

Implementation Strategy:

• Non-Breaking: Existing messages must continue to show actions persistently unless the new prop is passed.
• CSS: Use properties that preserve document flow (avoid display: none) to meet the "no layout shift" requirement.

Acceptance Criteria

• Feature is opt-in and does not change default persistent action visibility.
• Actions appear on hover/focus and disappear on blur/leave.
• Zero layout shift occurs when actions toggle visibility.
• Actions are reachable and visible via keyboard Tab navigation.