DAP namespace settings for .com

earlier iterations of Model Selection, we enabled namespace model selection for existing GA features for customers on Gitlab.com #526009 (closed) and instance level model selection for DAP. This issue is to close the gap and enable namespace level model configuration for DAP on GitLab.com.

• Add a new feature flag (FF) for user model selection on .com.
• Add Duo agent platform to duo_agent_platform https://gitlab.com/gitlab-org/gitlab/-/blob/da3c9de796e6ca18cce9b52904864f1e184c2a34/ee/app/models/concerns/ai/model_selection/features_configurable.rb
• Users should be able to pin a model for DAP on the namespace level.
• Priority:
  1. A namespace setting is assigned for the feature.
  2. User model selection if no namespace setting is assigned.

Definition of Done

Gitlab.com namespace administrators who are accessing DAP via the GitLab.com AIG/WFS can:

• pin one model for use with all of DAP
• choose which model to pin from among Gitlab offered GL AI Vendor models
  • models supported for this iteration are Claude 3.5 Sonnet, Claude 3.7 Sonnet, and Claude 4 Sonnet
• Take no action, and DAP will default to Claude 4 Sonnet