Share granular scope builder for personal access tokens

What does this MR do and why?

Part of #631824 (closed).

This MR moves the granular scope builder into one shared class. Behaviour does not change.

Two copies of the builder existed:

• lib/api/helpers/personal_access_tokens_helpers.rb
• app/graphql/mutations/users/personal_access_tokens/create.rb

Both turned {access, permissions, resources} inputs into unsaved Authz::GranularScope records. Both derived the namespace from the access level. Both guarded each group or project with Ability.allowed?(user, :read_boundary, resource).

Authz::GranularScope has no validation tying access level to namespace. The builders are the only thing keeping them consistent. OAuth applications, in the follow-up, need the same derivation with a different boundary rule per application owner. They would have become a third copy.

What changes

• New Authz::GranularScopes::Builder in app/models/authz/granular_scopes/builder.rb.
  • It takes normalised inputs and a boundary_rule: object. The rule responds to allowed_resource?(resource) and personal_projects_namespace.
  • It returns unsaved records in input order.
  • It raises Builder::ResourceNotAllowedError on the first refused resource.
  • It does not load records by ID, set organization_id, deduplicate or persist.
  • It is a plain object under app/models/authz because it changes no state, like Authz::Boundary.
• New Authz::GranularScopes::ReadBoundaryRule is the user-owned rule. It uses Ability.allowed?(user, :read_boundary, resource) and user.namespace. It is an object rather than a lambda because two behaviours are needed. The follow-up adds rules for group-owned and instance applications, which have no user.
• The REST helper keeps build_granular_scopes(current_user, inputs). Impersonation tokens still check read_boundary against the impersonated user. The helper loads groups and projects by ID, normalises, and calls the builder. It maps ResourceNotAllowedError to not_found! as before.
• The GraphQL mutation batch-loads from global IDs, syncs, and calls the builder. It maps the error to raise_resource_not_available_error! as before.

References

• Work item: #631824 (closed)
• Parent stream: #630327
• Follow-up that consumes both MRs: #631825
• Companion MR, the uniqueness check. The two MRs are independent and both target master: !259531 (merged)

How to set up and validate locally

Use a GDK instance. <token> is a personal access token with the api scope. The granular_personal_access_tokens flag is on by default.

1. Create a token with one selected_memberships entry naming one group and one project the user is a member of. Expect HTTP 201 and two scopes. Their namespaces are the group and the project's namespace.
2. Create a token with an entry naming a private project the user cannot read. Expect HTTP 404.
3. Create a token with a personal_projects entry. Expect HTTP 201 and a scope whose namespace equals the user's namespace.
4. Run the same creation through GraphiQL at /-/graphql-explorer. Use the personalAccessTokenCreate mutation with resourceIds for the same group and project. Expect the same two scopes as in step 1.

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