Draft: API endpoint for AI Gateway access token + details

What does this MR do and why?

API endpoint for AI Gateway access token + details

• Provides necessary detail for the LSP to make direct requests to the AI Gateway API for Code Suggestions.
• #433433 (closed)

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Before	After

How to set up and validate locally

• Enable the feature flag: echo 'Feature.enable(:code_suggestions_client_direct_to_gateway)' | rails c
• Generate a personal access token with the api scope at https://gdk.test:3443/-/profile/personal_access_tokens (this request can also be authenticated with an oauth access token but that is harder to generate locally for testing)
• curl request to endpoint:

  curl --request POST \
   --url https://gdk.test:3443/api/v4/code_suggestions/token_and_usage_details \
   --header 'Content-Type: application/json' \
   --header 'Authorization: Bearer $YOUR_GITLAB_PAT'

• response looks like this:

  {
    "access_token_expires_in": 3600,
    "created_at": 1701580530,
    "ai_gateway_base_url": "https://codesuggestions.gitlab.com",
    "ai_gateway_code_completion_model_family": "vertex_ai",
    "ai_gateway_code_generation_model_family": "vertex_ai",
    "ai_gateway_request_headers": {
      "X-Gitlab-Instance-Id": [
        "d9c14746-1da5-442b-a8a4-6e9d3ce24c53"
      ],
      "X-Gitlab-Global-User-Id": [
        "nLiw60WLCUJP5m1WNn+2VsWnHrfykMeDGzZcC7q27VQ="
      ],
      "X-Gitlab-Host-Name": [
        "gdk.test"
      ],
      "X-Gitlab-Realm": [
        "saas"
      ],
      "X-Gitlab-Authentication-Type": [
        "oidc"
      ],
      "Authorization": [
        "Bearer eyJ0e..."
      ],
      "Content-Type": [
        "application/json"
      ],
      "User-Agent": [
        "curl/8.1.2"
      ],
      "X-Gitlab-Saas-Namespace-Ids": [
        ""
      ]
    }
  }

MR acceptance checklist

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