Switch to Anthropic for ZeroShot agent

What does this MR do and why?

Switch to Anthropic for ZeroShot agent

This changes the client we inject for the ZeroShot agent to Anthropic and fixes a small bug with a namespaced translation string.

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Before	After

How to set up and validate locally

1. Set up the local environment for Anthropic https://docs.gitlab.com/ee/development/ai_features.html#test-ai-features-locally
2. Enable the feature flag Feature.enable(:gitlab_duo)
3. Test it with the GraphQL mutation and message history:

query messages {
  aiMessages {
    nodes {
      id
      requestId
      content
      errors
      role
    }
  }
}

mutation chat {
  aiAction(input:{chat:{resourceId:"gid://gitlab/Issue/645", content:"Summarize the comments"}}) {
    requestId
    errors
  }
}

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.