Adds gitlab models to feature setting query

What does this MR do and why?

Adds gitlabModel, defaultGitlabModel and validGitlabModels to the feature setting query

How to set up and validate locally

1. You will need AIGW running

2. On the Duo self-hosted admin area, update some feature settings.

3. With :instance_level_model_selection enabled, perform the following query

query getAiFeatureSetting {
  aiFeatureSettings {
    nodes {
      feature
      title
      mainFeature
      provider
      releaseState
      selfHostedModel {
        id
        releaseState
      }
      validModels {
        nodes {
          id
          name
          model
          modelDisplayName
          releaseState
        }
      }
      validGitlabModels {
        nodes {
        	name
        	ref 
        }
      }
      defaultGitlabModel {
        name
        ref
      }
      gitlabModel {
        name
        ref
      }
    }
  }
}

Example response:

{
  "data": {
    "aiFeatureSettings": {
      "nodes": [
        {
          "feature": "code_generations",
          "title": "Code Generation",
          "mainFeature": "Code Suggestions",
          "provider": "vendored",
          "releaseState": "GA",
          "selfHostedModel": null,
          "validModels": {
            "nodes": [
              ...
            ]
          },
          "validGitlabModels": {
            "nodes": [
              {
                "name": "Claude Sonnet 4.0 - Anthropic",
                "ref": "claude_sonnet_4_20250514"
              },
              {
                "name": "Claude Sonnet 4.0 - Vertex",
                "ref": "claude_sonnet_4_20250514_vertex"
              },
              {
                "name": "Claude Sonnet 3.7 - Anthropic",
                "ref": "claude_sonnet_3_7_20250219"
              },
              {
                "name": "Claude Sonnet 3.7 - Vertex",
                "ref": "claude_sonnet_3_7_20250219_vertex"
              },
              {
                "name": "Claude Sonnet 3.5 - Anthropic",
                "ref": "claude_3_5_sonnet_20240620"
              },
              {
                "name": "Gemini 2.5 Flash - Vertex",
                "ref": "gemini_2_5_flash_vertex"
              }
            ]
          },
          "defaultGitlabModel": {
            "name": "Claude Sonnet 4.0 - Anthropic",
            "ref": "claude_sonnet_4_20250514"
          },
          "gitlabModel": {
            "name": "Claude Sonnet 4.0 - Anthropic",
            "ref": "claude_sonnet_4_20250514"
          }
        },
....

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #565711 (closed)