feat: implement project hierarchy API (parents, subtree)

[gtema]

Parent issue: #938

Implement the project hierarchy endpoints currently missing in the Rust Keystone.

Endpoints to implement

Endpoint	Actions
/v3/projects/{project_id}/parents	GET (list parents)
/v3/projects/{project_id}/subtree	GET (list subtree), POST (create subtree), DELETE (delete subtree)

Reference

Python Keystone: project hierarchy API

Tempest tests: tempest.api.identity.v3.test_projects.test_project_create_with_parent, test_list_projects_with_parent, and related hierarchy tests.

Notes

• See Project CRUD/negative gaps beyond hierarchy (tempest test_projects, test_projects_negative, test_default_project_id, test_list_projects) #1040 for separate project CRUD failures (update_desc, update_enable, update_name, etc.) that are distinct from this hierarchy gap
• Follow existing CRUD patterns (backend.rs convention)
• Policy enforcement via OPA Rego in policy/ directory
• Unit tests: valid auth + positive/negative policy, invalid auth (per AGENTS.md)