Fix tag item association

[Herafia]

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes !45879
• Assigning an item to a tag from the tag's details page no longer worked, without generating an error.

Regression : The file front/tag.form.php, which handled this association, had been accidentally deleted in an unrelated commit. The request was then processed by the generic mechanism, which created a tag instead of establishing the association.

Fix : re-enable correct processing of the tag/item association.

Screenshots (if appropriate):

[Rom1-B]

see comment