fix(a11y): don't auto-open dropdown on focus, hide decorative chevron

[pringelmann]

Summary

Fixes two WCAG findings on the combobox reported against the use management "New Account" dialog (nextcloud-gmbh/customer-feature-requests#1587).

• WCAG 3.2.1 - tabbing into the search input used to auto-open the dropdown, which jumps the screen reader into the listbox and announces an option before the field label. Users never hear which field they're in. Dropped the focus side-effect; Space/enter/down arrow still open the dropdown, clicks still work.
• WCAG 4.1.2 - the chevron button exposed a useless "options list" accessible name via aria-labelledby. It's tabindex="-1" and the combobox input already exposes the open state, so it's purely decorative. Marked it aria-hidden="true" and dropped remaining aria tags. Mouse users still get the click target.

Behavior changes

Worth a minor bump, not a patch:

• Code relying on "focus opens the dropdown" as a side-effect needs to open it explicitly or use a keyboard/click gesture.
• Tests querying aria-labelledby/aria-controls/aria-expanded on the button will break.

No props, events, or methods changed.

Follow-ups

Once released:

1. nextcloud-vue: migrate NcSelect on main from upstream vue-select back to @nextcloud/vue-select, bump to this version
2. server: bump @nextcloud/vue

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.29%. Comparing base (4ca0c0b) to head (74d22e6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #73      +/-   ##
==========================================
- Coverage   91.31%   91.29%   -0.02%     
==========================================
  Files          10       10              
  Lines         449      448       -1     
  Branches      145      146       +1     
==========================================
- Hits          410      409       -1     
  Misses         33       33              
  Partials        6        6              

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