fix(GlBreadcrumb): Fix items watcher behavior

What does this MR do?

Before this change, even with auto-resize set to false, a change of the items props would trigger a watcher that would then call the auto-resize logic, ignoring the value of auto-resize 🐛

Fixes #2906 (closed).

Screenshots or screen recordings

Before	After
recording_1727779290588	recording_1727779333631

How to reproduce bug and fix locally

• Start yarn storybook
• Go to https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/base-breadcrumb--collapsed-items&args=autoResize:!false
• In the Controls tab, edit the items prop (by clicking "Raw" and editing the JSON)
• The bug (on main): The items watcher triggers a resize, collapsing items, even though auto-resize is disabled.
• The fix (this branch): The items watcher returns early, doing nothing (but marking all items as "fitting").

Integration merge requests

Not needed here.

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