Add dap_workspace_agents feature flag

Alexander Chueshevrequested to merge
duo/feature/dap-workspace-agents into master

What does this MR do and why?

Add dap_workspace_agents feature flag

Gates attaching customer-defined workspace agents to foundational flows as subagents. The feature itself lives in the AI Gateway, so Rails only pushes the flag state in the x-gitlab-enabled-feature-flags header. The push is wired up in the same commit because a flag definition with no usage fails the rspec:feature-flags job.

Related to #622600 (closed)

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