[DSpace-CRIS] Implement detailed error handling for custom url conflicts

[FrancescoMolinaro]

References

Fixes: #5078
Relates to: #4814

Description

Implemented error handling on item page for custom url conflict.
Added direct link to administrative edit for item correction.

Instructions for Reviewers

Implemented a new component shown on the item page when a custom URL lookup returns a 500 error, which indicates that multiple items share the same dspace.customurl value.
Uses SearchService.search() with a dspace.customurl filter to find all items with the conflicting value, then renders a direct edit link (/edit-items/:FULL) for each result so administrators can immediately navigate to and fix the affected items.
Placed inside the item page template when itemRD.statusCode === 500 and the resolved route param is not a UUID (i.e. it was a custom URL lookup).

Step to reproduce:

• Define an identical custom url on multiple items
• Navigate to the item page of one of the items

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