[JENKINS-72988] validate displayname against items in the same ItemGroup

[mawinter69]

The check if a displayname collides with an existing job name or displayname was always performing the check against the root. So if one wanted to use a displayname for a job inside a folder the check would wrongly fail if the name is used by a something at root level and not fail when it is used by another item in the folder

depends on #9150

See JENKINS-72988.

Testing done

Manual testing

Proposed changelog entries

• Validate display name only against items in the same ItemGroup.

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N/A

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[daniel-beck]

depends on #9150

This PR looks like it should go through quicker (straightforward bug fix, no dependency on downstream PRs), so a different order would be better IMO.

[mawinter69]

depends on #9150

This PR looks like it should go through quicker (straightforward bug fix, no dependency on downstream PRs), so a different order would be better IMO.

the change has no real effect without the other PR. The doCheckDisplayName is only called in AbstractProject (which is not a TopLevelItem so it can't really call the new method), otherwise it is only called from plugins afaik. Maybe it makes sense to merge the 2 changes as the new method in the TopLevelItemDescriptor is what should be used instead of doCheckDisplayName

[timja]

LGTM

[timja]

/label ready-for-merge

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This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!