Fix New Item validation not disabling submit button

[janfaracik]

Fixes #16416

This PR fixes the OK button not being disabled if there's a duplicate name. The button should be disabled if there is any error on the page.

#16416 (comment) mentions an adjustment to ATH is needed - that's covered in jenkinsci/acceptance-test-harness#2572.

Testing done

Before

Screen.Recording.2026-01-05.at.13.48.34.mov

After

Screen.Recording.2026-01-05.at.13.48.04.mov

Proposed changelog entries

• Fix New Item validation not disabling submit button

Proposed changelog category

/label bug

Proposed upgrade guidelines

N/A

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Desired reviewers

@jenkinsci/sig-ux @daniel-beck

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[daniel-beck]

This introduces a regression whereby typing in the "Copy from" field resets the "Item name" form field validation error.

Before:

After:

[janfaracik]

This introduces a regression whereby typing in the "Copy from" field resets the "Item name" form field validation error.

Good spot, very odd. I've removed a line from copyFromFieldEvent in which it was setting the empty line validation on the name input - not exactly sure why that was there considering it relates to a different field/why it wasn't being triggered before. Haven't noticed any issues.

[github-actions]

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