Prevent label dropdown from overflowing

What does this MR do and why?

Prevents the sidebar labels button from overflowing the available space.

While the outcome is the same, this impacts all issuable sidebar use cases (issues, epics, MRs)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create a long label (e.g. Category:Subscription Management)
2. Apply the label alongside other labels to an Issue, MR, or Epic, ensuring the long label is alphabetically first so it shows up in the button text

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Closes #379031 (closed)