feat: added a _position.yaml for vertex ai provider

[Riddhimaan-Senapati]

Summary

The vertex model provider didn't have a _position.yaml files like other providers so I decided to add one.

Fixes #13368

Screenshots

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Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[Riddhimaan-Senapati]

@crazywoola Can you tell me what is wrong with the _position.yaml file that I added.

[Riddhimaan-Senapati]

@crazywoola Now can you review my PR? I got rid of the weird linting error.

[crazywoola]

Please see #13399 (comment) as well
For changes regarding model runtime and tools should move changes to that repo.
@Riddhimaan-Senapati