Add dap_parallel_subagents feature flag

What does this MR do and why?

Adds a new experiment feature flag, dap_parallel_subagents, and forwards it to the AI gateway from the Duo Workflow API helpers. The flag is pushed unconditionally with current_user as the actor, alongside the other Duo Agent Platform flags already handled in push_feature_flags.

On its own this changes no behavior in the Rails app. Nothing here reads the flag; it only becomes visible to the AI gateway so the Duo Workflow Service can gate parallel subagent execution on it. The flag is off by default.

References

• Feature issue: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist#2070
• Rollout issue: #618641

How to set up and validate locally

bundle exec rspec ee/spec/lib/api/helpers/duo_workflow_helpers_spec.rb

You can also toggle the flag in the rails console with Feature.enable(:dap_parallel_subagents) and confirm that the x-gitlab-enabled-feature-flags response header on a Duo Workflow API request includes it.

Feature flag

• Name: dap_parallel_subagents
• Type: experiment
• Defaults to off (default_enabled: false)