fix(Breadcrumbs): expose collapsed-items count and aria-expanded on expander

[sridhar-3009]

Description

Collapsed breadcrumb items are invisible to screen readers: the ellipsis hardcodes aria-hidden="true" and is non-interactive, so truncated levels are unreachable by AT and there is no disclosure path to reveal them.

This PR makes the ellipsis an optional disclosure toggle and exposes the hidden-item count:

• BreadcrumbsEllipsis gains an interactive prop. When set, aria-hidden is no longer hardcoded and the attrs expose aria-expanded (false collapsed / true expanded), an aria-label announcing the hidden count (new locale key Breadcrumbs.expand, e.g. "Show 3 more breadcrumbs"), role="button"/tabindex="0" when not rendered as a native button, and click + Enter/Space activation. The accessible name stays stable while the disclosure is open. Non-interactive behavior is unchanged.
• BreadcrumbsRoot tracks disclosure state (expanded) and hiddenCount in its context; when expanded, the visibility watcher reveals all collapsed content and keeps the ellipsis visible so it can serve as the collapse toggle.
• BreadcrumbsItem now exposes aria-hidden="true" in its slot attrs while collapsed, so renderless consumers that keep collapsed items rendered still remove them from the a11y tree.
• Added Breadcrumbs.expand to the built-in English messages.

Tests

• Interactive ellipsis exposes aria-expanded, count label, role/tabindex, and activation handlers; native as="button" omits the redundant ones
• Click and Enter/Space toggle aria-expanded
• Locale override for the disclosure label
• Expanding reveals all collapsed items and hiddenCount returns when collapsed again
• Collapsed items expose aria-hidden="true" in slot attrs

All 76 Breadcrumbs tests pass; lint and typecheck clean.

Closes #614

[github-actions]

✅ Changeset found — this change will be included in the next release. Thanks!