feat(user-message): support grouped multi-attachment messages

[TarunAdobe]

Add swc-user-message-attachment: a minimal, presentation-only tile (media/card, no dismiss, no click behavior) kept as a separate component from swc-upload-artifact so the compose-time and sent-message tiles can diverge independently.

Extend swc-user-message with type="attachments": media attachments lay out in a 4-column grid, card attachments stack full-width beneath it, and overflow beyond 4 media tiles collapses behind a show all/less disclosure (same open/aria-expanded/aria-controls shape as swc-message-sources), firing swc-user-message-toggle. Grouping and slot-routing are driven by a MutationController watching the light DOM, following the same pattern as swc-response-status.

Adds unit tests for grouping/overflow/toggle behavior, an ARIA snapshot test, stories, and docs.

Description

Motivation and context

Related issue(s)

• fixes [Issue Number]

Screenshots (if appropriate)

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Author's checklist

• I have read the CONTRIBUTING and PULL_REQUESTS documents.
• I have reviewed at the Accessibility Practices for this feature, see: Aria Practices
• I have added automated tests to cover my changes.
• I have included a well-written changeset if my change needs to be published.
• I have included updated documentation if my change required it.

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Reviewer's checklist

• Includes a Github Issue with appropriate flag or Jira ticket number without a link
• Includes thoughtfully written changeset if changes suggested include patch, minor, or major features
• Automated tests cover all use cases and follow best practices for writing
• Validated on all supported browsers
• All VRTs are approved before the author can update Golden Hash

Manual review test cases

• Descriptive Test Statement

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• Descriptive Test Statement

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Device review

• Did it pass in Desktop?
• Did it pass in (emulated) Mobile?
• Did it pass in (emulated) iPad?

Accessibility testing checklist

Required: Complete each applicable item and document your testing steps (replace the placeholders with your component-specific instructions).

• Keyboard (required — document steps below) — What to test for: Focus order is logical; Tab reaches the component and all interactive descendants; Enter/Space activate where appropriate; arrow keys work for tabs, menus, sliders, etc.; no focus traps; Escape dismisses when applicable; focus indicator is visible.

  1. Go here
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  3. Expect this result

• Screen reader (required — document steps below) — What to test for: Role and name are announced correctly; state changes (e.g. expanded, selected) are announced; labels and relationships are clear; no unnecessary or duplicate announcements.

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[changeset-bot]

⚠️ No Changeset found

Latest commit: 7ed638e

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