Add AI Gateway development mode configuration

Problem Statement

Engineers manually manage several environment variables in gdk.yml when working with AI Gateway. Switching between local, staging, or self-hosted models requires updating multiple variables and running Rails console commands.

Impact

This manual process is error-prone, time-consuming, and inconsistent across engineers.

Proposal

Add this config to gdk.yml:

gitlab_ai_gateway:
  environment: 'local' # 'local' or 'staging'
  use_self_hosted_models: false

GDK will automatically manage the environment variables based on these settings. Engineers only need to change one config instead of managing multiple variables.

Impacted categories

The following categories relate to this issue:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.