Add the settings evaluation context builder

What does this MR do and why?

Adds Govern::PolicyEngine::SettingsContext. It builds the input document a settings evaluation sends to the policy engine: the envelope with schema version, trigger, and organization, the scope dimensions the compiled scope_rego reads, and a flat map of the governable project settings. Govern::PolicyEngine::ApprovalSettings holds the approval settings whose project column has another name, and sometimes the opposite value.

Every value comes from the project's own row, before group or instance resolution. A project that complies only because its group is strict still counts as non-compliant on its own row, which a converging policy has to act on.

Nothing calls the builder yet. The evaluation service of https://gitlab.com/gitlab-org/gitlab/-/work_items/630148 consumes it next. The code comes from the prototype in !259574, now with specs. The spec validates the built document against the GLAZ context schema, vendored under ee/spec/fixtures/govern/. gitlab-org/auth/glaz!284 (merged) registers the same schema in the engine. The default trigger id is a string constant in this class until the gem vocabulary includes the settings trigger ids. No changelog entry, because security_policies_v2 defaults to off.

This MR sits on top of !259956 (merged) and targets its branch. It retargets to master once that one merges.

References

• Issue: https://gitlab.com/gitlab-org/gitlab/-/work_items/630151
• Epic: https://gitlab.com/groups/gitlab-org/-/work_items/23462

Screenshots or screen recordings

Backend-only change, no UI.

Before	After

How to set up and validate locally

1. In the Rails console, build the context for any project:

   Govern::PolicyEngine::SettingsContext.build(project: Project.first)

2. Compare the project.settings keys with the key list in the emits exactly the governable project settings example of ee/spec/lib/govern/policy_engine/settings_context_spec.rb.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.