fix(NcCheckboxRadioSwitch): use reactive state for group checkbox toggle

[pringelmann]

The array/group checkbox path in onToggle read input.checked from the DOM via getInputsSet(). On keyboard spacebar, the browser toggles input.checked before firing the change event, so the DOM-read logic inverted the intended action and emitted the stale modelValue.

Replace DOM queries with reactive-state reads (this.isChecked and this.internalModelValue) which correctly reflect the pre-toggle state regardless of the event source.

Fixes #8321

☑️ Resolves

• Fix #…

🖼️ Screenshots

🏚️ Before	🏡 After
B	A

🚧 Tasks

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🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable
• 2️⃣ Backport to stable8 for maintained Vue 2 version or not applicable

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.01%. Comparing base (baefe78) to head (66b3d55).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8324      +/-   ##
==========================================
+ Coverage   52.40%   53.01%   +0.60%     
==========================================
  Files         104      104              
  Lines        3393     3388       -5     
  Branches      989      988       -1     
==========================================
+ Hits         1778     1796      +18     
+ Misses       1359     1344      -15     
+ Partials      256      248       -8     

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