fix(GlBreadcrumbs): Add medium variant of collapse calculation

What does this MR do?

The small size dropdown toggle measures 24px, 26px with the boarder. It's the same for the dropdown toggle, when the dropdown is of size medium. Despite the same toggle size, breadcrumb-item-sm that includes the dropdown toggle measures 46.328px in width, while breadcrumb-item-md with the same toggle measures 47.05px. I believe that the difference in these two widths is what's causing the flickering between the two states.

It's quite hard to reproduce. I was able to do it on the GDK under this url: http://gdk.test:8080/gitlab-org/iglu/-/tree/master/src/test/java/com/example. In that edge case, the first item width when it's still a text is 45.16px for the breadcrumb-item-md with 24.12px of the anchor element inside it.

The problem:

• Original text item width: 45.16px
• Dropdown replacement width: 47.05px
• Current buttonWidth: 40px (too small)

The flickering cycle:

• Algorithm uses 40px, thinks dropdown will fit → collapses item
• Actual dropdown renders at 47.05px (larger than original 45.16px text)
• Now it doesn't fit → tries to uncollapse
• Back to 45.16px text → cycle repeats

Solution: Set buttonWidth to 48px for medium size breadcrumbs.

Screenshots or screen recordings

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Flickering content in the expandable section

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Before	After
Screen_Recording_2025-07-09_at_15.36.12	Screen_Recording_2025-07-09_at_15.28.32

Integrations

• GitLab: gitlab-org/gitlab!197150 (closed)
• CustomersDot: mr_url
• Duo UI: mr_url
• Status Page: mr_url
• Docs: mr_url
• Switchboard: mr_url

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Related to #2915 (closed)