Move explain-vulnerability button into header actions
What does this MR do and why?
Moves the "Explain Vulnerability with AI" feature from the vulnerability-details section into the split-button within the header. This is to follow the finalised designs: #464636[design_1718663433524.png] (comment 1957907897)
![#464636[design_1718663433524.png] (comment 1957907897)](https://gitlab.com/gitlab-org/gitlab/-/issues/464636/designs/design_1718663433524.png#note_1957907897){kind=link}
MR acceptance checklist
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Screenshots or screen recordings
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How to set up and validate locally
- Enable the related FF:
echo "Feature.enable(:explain_vulnerability_tool)" | rails c - Verify that the "Explain Vulnerability" card does not render within the details section
- Verify that the button renders within the header
Related to #468009 (closed)
Edited by David Pisek