fix(federated-sharing): avoid duplicate shares - #63320

Summary

Reproduction steps:

• admin shares folder 'Test' with local user Bob
• admin shares 'Test' with remote user User1 by typing his name in the user selecting it
• Bob attempts to share the way but gets an error ( Expected behaviour ✅)
• Bob manually types user1@http://nextcloud2.local/ ( Works 🚫 )

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
• 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)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI