Allow AgenticChatModelMetadataService to use developer selectable models

What does this MR do and why?

This merge request (MR) updates AgenticChatModelMetadataService to use FeatureSetting.decorate instead of ModelDefinitionResponseParser when validating user-selected models. This MR is needed because the current implementation does not take into account developer selectable models introduced in !210909 (merged) when checking if a model is valid.

References

• Feature enabling developer selectable models: !210909 (merged)
• Parent issue: #581701

Screenshots or screen recordings

Before	After

How to set up and validate locally

The following steps require a working GitLab Development Kit (GDK) available locally:

1. Set up AI Gateway in your GDK. In addition, make sure to have this in your env.runit file:

   export FETCH_MODEL_SELECTION_DATA_FROM_LOCAL=1

2. Configure AI Gateway to return some developer-only models:

   Edit <gdk_dir>/gitlab-ai-gateway/ai_gateway/model_selection/unit_primitives.yml and add developer models for duo_agent_platform:

   - feature_setting: "duo_agent_platform"
     unit_primitives:
       - "duo_agent_platform"
     default_model: "claude_sonnet_4_20250514_vertex"
     selectable_models:
       - "claude_sonnet_3_7_20250219"
       - "claude_sonnet_4_20250514"
       - "claude_sonnet_4_20250514_vertex"
     dev:
       selectable_models:
         - "claude_haiku_4_5_20251001"  # Dev-only model
         - "gpt_5_codex"                 # Dev-only model
       group_ids:
         - 1000000  # GitLab Duo group ID (or use your test group ID)

3. Bypass the team member check:

   Edit ee/lib/gitlab/graphql/representation/model_selection/feature_setting.rb:

   def use_dev_overrides?
     return false unless Gitlab::Saas.feature_available?(:gitlab_com_subscriptions)
     return false unless true # current_user&.gitlab_team_member? # <-- Change this line

4. Ensure you're on GitLab.com.

5. Check that developer-only models are accessible for Agentic Chat:

   1. Go to GitLab Duo group (group ID: 1000000c): http://gdk.test:3000/gitlab-duo

   2. Open GitLab Duo Chat and enable Agentic mode.

   3. Verify developer-only models are accessible in the UI:

      If this MR works as intended, you should see the developer models you listed in unit_primitives.yml along with the regular models in the model selection dropdown menu:

      

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 #581701