fix(a11y): keep visible text in Setup Provider button's accessible name (#14569)

The Setup Provider CTA on the Language Model field forwarded the field
label as its sole accessible name (aria-labelledby -> "Language Model
required"), so its visible text "Setup Provider" was absent from the
name. That fails WCAG 2.5.3 (Label in Name, level A): screen reader users
hear the field name rather than the action, and speech-input users
saying "click Setup Provider" match nothing.

Unlike the combobox branch of the same trigger — whose visible text is
its *value*, so the field label alone is the correct name — this branch
is a plain button whose visible text is its label. Compose the button's
own text first and keep the field label after it, so the name leads with
what the user reads while still identifying which field the CTA belongs
to. Same composition for the literal aria-label fallback.

This also restores the stated intent of #14444, whose QA item 7 called
for "the field's label if the button is icon-only, or the visible button
text if present".