Fix segment explorer file pill accessibility (#92608)
### What?
Make the segment explorer file pills keyboard-accessible by rendering
them as buttons and preserving their pill styling.
### Why?
The pills launch files in the editor, but they were rendered as
clickable spans, so they were not exposed as interactive controls for
keyboard and assistive technology users.
### How?
- replace the clickable span with a button element
- add an accessible label and hide decorative icons from assistive
technology
- add pointer and focus-visible styles so the control remains
recognizable and keyboard accessible
Closes NEXT-
Fixes #
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