chore: make vertex the default provider for Sonnet-backed features
What does this merge request do and why?
Part of https://gitlab.com/gitlab-org/gitlab/-/issues/560301
How to set up and validate locally
Numbered steps to set up and validate the change are strongly suggested.
Merge request checklist
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Tests added for new functionality. If not, please raise an issue to follow up. -
Documentation added/updated, if needed. -
If this change requires executor implementation: verified that issues/MRs exist for both Go executor and Node executor or confirmed that changes are backward-compatible and don't break existing executor functionality.