Adding completions to google vertex provider only for anthropic based models

[arafatkatze]

Linear Issue

This PR adds support for completions support for Google Vertex but only for Anthropic based models(NOTE: Gemini and other models don't support completions when used with the google provider).

Sense of Urgency: This is a P0 because of enterprise requirements so I would appreciate a fast approval and merging.

Test plan

• Run this branch for sourcegraph instance -> https://github.com/sourcegraph/sourcegraph/pull/63282
• Ask @arafatkatze to dm you the siteadmin config to make things work
• Check the logs and play with completions