VEL-7227 Add feedback message for OSS::Country selector

[JulienVannier66]

What does this PR do?

This PR adds feedback message for OSS::CountrySelector component, allowing display of error, warning, or success messages below the input with corresponding border styling.

Component Updates:

• Added @feedbackMessage argument accepting a FeedbackMessage type with type and value properties
• Implemented computedClasses getter to dynamically apply styling based on feedback message type
• Added feedback message display in template with color-coded text

Styling:

• Added .country-selector-container--error, --warning, and --success modifiers
• Each modifier applies corresponding input border styles

Testing:

• Added comprehensive test coverage for all feedback message scenarios
• Tests validate message display, border styling, and reactive updates
• Coverage includes all allowed types (error, warning, success) and edge cases (null, undefined)

Documentation:

• Added Storybook story demonstrating error feedback message usage
• Updated demo template with examples of feedback message integration

What are the observable changes?

Screencast.from.2026-02-03.18-03-05.webm

Good PR checklist

• Title makes sense
• Is against the correct branch
• Only addresses one issue
• Properly assigned
• Added/updated tests
• Added/updated documentation
• Migrated touched components to Glimmer Components
• Properly labeled

[linear]

VEL-7083 [creators-web] New billing-information/identity/form component

VEL-7227 Add feedbackMessage for OSS::CountrySelector

[aprentout]

LGTM :)