fix: show a single duplicate-name error during file creation

[mvanhorn]

• Resolves: #

Summary

Keep validity as the single source for the inline NcTextField error and retain the input's custom validity state so form submission, including Enter-key submission, remains blocked for duplicate and otherwise invalid names. Stop proactively invoking native validity reporting when reactive validation changes, which removes the transient browser tooltip without weakening the existing constraint. Add focused component coverage around the duplicate-name state, verifying the inline error, disabled Create action, retained native invalid state, and absence of a reportValidity() call.

The web file-creation dialog currently displays the duplicate-name validation twice: once as the intended red NcTextField helper text and once as a browser-native validation tooltip. NewNodeDialog.vue creates both channels by binding validity to the field's error/helper props while also calling reportValidity() after setting a custom validity message. The issue is reproducible by entering the name of an existing file or folder in the same directory. The desired behavior is one persistent inline error with creation still blocked until the name becomes valid.

Fixes #62686

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
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• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
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