Add feature flag for DAP Impact v1 analytics aggregation engine

What does this MR do and why?

This MR introduces the dap_impact_v1 feature flag to gate the experimental duoWorkflows field on the GraphQL Analytics type. This field powers an aggregation engine that will provide analytics for GitLab Duo Agent Platform flows. Since the DAP Impact v1 analytics feature is still under active development, gating it behind a flag lets the team continue merging incremental work without exposing unfinished analytics to users. While the flag is disabled, querying the field returns a "resource not available" GraphQL error.

References

• Feature issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/622366

How to set up and validate locally

1. In a rails console, enable the feature flag: Feature.enable(:dap_impact_v1).
2. Query the duoWorkflows field via GraphQL on a group and confirm it returns data as expected.
3. Disable the flag: Feature.disable(:dap_impact_v1).
4. Re-run the same query and confirm it now returns a "resource not available" GraphQL error.

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