Send duo_agent_platform as the Duo Workflow unit primitive header

What does this MR do and why?

Related to: https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/work_items/1269

This MR updates our previous flows sending a duo_workflow_execute_workflow unit primitive which had no add_ons to now be duo_agent_platform.

duo_agent_platform covers every relevant add-on:

duo_pro duo_enterprise self_hosted_dap duo_core gitlab_credits

For reference, here's the legacy primitive's definition in the Cloud Connector gem. Note that it has no add_ons key:

Use duo_agent_platform as the Cloud Connector unit primitive header for Duo Workflow requests. Previously we use the duo_workflow_execute_workflow primitive for requests but that has no add_ons for cloud connector usage.

Validation

bin/rspec ee/spec/lib/gitlab/duo_workflow/client_spec.rb

No UI changes.