[Fixes #14437] Fixes keywords lost on style updates

[sijandh35]

Fixes #14437

Checklist

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[gemini-code-assist]

Code Review

This pull request updates the GeoServer keyword synchronization logic to merge incoming keywords with existing ones instead of replacing them. It introduces a --skip-keywords option to the updatelayers command and adds a new importgskeywords parameter to control when keywords are imported. Additionally, corresponding unit tests have been added to verify the merging behavior. The feedback suggests adding a fallback empty list when converting instance.keyword_list() to a set to prevent potential TypeError exceptions if it returns None.

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