fix: duplicate dashboard widget

[mykh-hailo]

• Resolves: Dashboard widget can be added multiple times #59146

Summary

This PR fixes a dashboard layout edge case where duplicate widget IDs can be sent to the layout update endpoint.
Even though the UI only allows selecting a widget once, crafted requests could still submit duplicates (e.g. layout[]=files&layout[]=files). This caused duplicated entries to be persisted.

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

[susnux]

Thank you for your pull request!
I have just one comment:

[susnux]

/backport to stable33

[susnux]

/backport to stable32

[mykh-hailo]

@leftybournes , @Altahrim , @CarlSchwan , @provokateurin
I'd appreciate it if you review this PR at your convenience.

[welcome]

Thanks for your first pull request and welcome to the community! Feel free to keep them coming! If you are looking for issues to tackle then have a look at this selection: https://github.com/nextcloud/server/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22

[github-actions]

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