Add dependency_firewall_risk_severity_rule feature flag

Hannah Bakerrequested to merge
627719-risk-severity-rule-flag into master

What

First slice of the risk_severity DFW rule builder work. Defines the dependency_firewall_risk_severity_rule feature flag and pushes it to the frontend from both the project- and group-level policy editor controllers, matching the existing dependency_firewall_phase2 wiring.

The backend rule type (schema, evaluator, persistence/GraphQL, dashboard label) already shipped as part of the risk_severity series under gitlab-org/gitlab#617114. This slice only adds the flag that will gate the new rule-builder UI, landing in a follow-up MR stacked on this one.

Why split this way

Splitting the POC (draft MR !253663 (closed)) into small, individually reviewable MRs. This one is deliberately inert on its own — nothing reads the flag yet — so it's the safest possible first slice to land.

MRs in this series

  1. This MR — feature flag definition + push
  2. Shared rule-type constants/utils (stacked on this MR)
  3. New risk_severity rule builder component (stacked on 2)
  4. Wiring into the rule-type picker/list (stacked on 3)

None of these merge into each other; each targets its own branch and is reviewed independently. poc-dfw-risk-severity-rule (!253663 (closed)) remains the reference branch proving the whole thing works end-to-end.

Related

Relates to https://gitlab.com/gitlab-org/gitlab/-/work_items/627719

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