feat: add authz schema loading

[rodmgwgu]

Problem

Discovered resources are still opaque bytes. They need to be parsed into typed schema documents, with the source identity and content digest that later phases and the provenance tables depend on (ADR 0018 §1).

Approach

The load phase, plus the shared vocabulary the remaining phases are written against.

• openedx_authz/engine/schema/loading.py — SchemaLoader
• openedx_authz/engine/schema/types.py — the schema dataclasses
• openedx_authz/engine/schema/exceptions.py — the pipeline exception hierarchy
• openedx_authz/constants/schema.py — SchemaOriginKind
• openedx_authz/tests/schema/{factories,test_loading,test_types}.py

types.py and exceptions.py land whole rather than sliced across the stack: they are the shared vocabulary, and splitting them would make every later PR conflict on the same two files. Some names in them (CompiledSchema, CompiledDefinition, RelationshipSource, and the compile/validate/apply errors) are unused until later PRs.

Manual testing instructions

pytest openedx_authz/tests/schema/

Rollback plan

Revert this PR. Nothing consumes the loader yet, so the revert is inert.

Retro compatibility

No authorization behavior changes. No models, no migration, no automatic code path.

AI Usage

Kiro was used to assist on feature planning and implementation. Implementation was done step by step with human guidance and validation, based on the ADRs.

---

Stack (2/8) — #446 split into reviewable pieces. Bases chain bottom-up; merge in order.

1. feat: add authz schema discovery #474 — schema discovery
2. This PR — schema loading (base: rod/authz-schema-discovery)
3. rod/authz-schema-compiler — schema compilation
4. rod/authz-schema-validator — schema validation
5. rod/authz-schema-models — definition models + migration
6. rod/authz-schema-renderer — policy renderer
7. rod/authz-schema-applier — schema applier
8. feat: add authz schema pipeline and load_authz_schema command #446 — pipeline + load_authz_schema command, version bump and changelog

Base is the previous branch; review only the incremental diff.

Merge checklist:

• Version bumped — deferred to 8/8 so the stack does not conflict on every restack
• Changelog record added — deferred to 8/8
• Documentation updated (not only docstrings) — covered by the already-merged ADRs
• Fixup commits are squashed away
• Unit tests added/updated
• Manual testing instructions provided
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets — depends on feat: add authz schema discovery #474; no migration

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[mariajgrimaldi]

A few comments for you to review! Thanks a lot for this :))

[mariajgrimaldi]

Could there be duplicates?

[rodmgwgu]

In theory yes, I'm adding a way to deterministically choosing the candidate:

1. First search for the distribution that actually has the discovered resource file
2. If more than one matches exists, sort and return the first one as a deterministic fallback (and log a warning about it.)

[mariajgrimaldi]

I wonder if these decisions should be recorded somehow. When we merge this stack, can we do like a reverse-engineering review to capture the deliberate decisions we're making here?

[mariajgrimaldi]

Should we use some kind of role_id to reference the role we're extending?

[rodmgwgu]

This also comes form the yaml definition as specified in the ADRs

[mariajgrimaldi]

The reference mentions role with a description of role ID, which is very straightforward. By itself not so much. We can update the ADRs and references if needed. Let me know what you think!

[mariajgrimaldi]

LGTM! Thanks so much for addressing my comments!

(The comments left to address are only nits so I'll leave my approval! :))