State pdp_digest explicitly; decline to publish a vocabulary mapping

flex-auth asked whether this engine should publish an action/target
mapping between the claim binding's vocabulary (secrets.kv.destroy,
{"id": "lane-openbao-root"}) and a policy package's (destroy, lane:...),
since their package makes no cross-check that a claim was approved for
the action being decided.

Answered no. A PIP asserting that one vocabulary's action means
another's would author policy semantics it does not own, over
vocabularies it does not own, and the failure mode is asymmetric: a wrong
mapping silently accepts a claim approved for a different action, which
is worse than no mapping. binding.pdp_digest is the correspondence and
sidesteps vocabulary entirely -- it compares the PDP's own digest to the
PDP's own digest, with no translation by anyone.

Implemented the part that was ours. pdp_digest was emitted only when
recorded, so a consumer could not distinguish "not issued against a
decision" from "we forgot to look". It is now always present and null in
that case, required-but-nullable in the schema, and documented as
something a PEP on a privileged lane must refuse. This engine states the
fact; enforcing the lane's policy stays with the consumer.

Both published examples were already contradicting the updated schema by
omitting the field -- the same fixture-versus-contract defect flex-auth
hit twice this week and that secrets-engine implemented. Fixed both, made
them cover the PDP-bound and unbound shapes so neither is inferred from
the other, and added tests/test_examples.py to validate every example
against the schema so the class cannot recur here. jsonschema added as a
dev dependency.

94 tests pass (6 new).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TvyJPAaVCGsVheVhcCwNND

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 411227@bnt-lap001
Assistant-Session: d566f6d3-bcaf-43c3-bc5e-3ddd0f64b535

tests/test_examples.py

@@ -0,0 +1,41 @@
+"""The published examples must satisfy the published schema.
+
+flex-auth shipped two defects in one day from fixtures that contradicted their
+own contracts, and secrets-engine built a validator against one of them. A
+contract whose examples contradict its prose will be implemented as its
+examples, so the examples are tested rather than trusted.
+"""
+
+import json
+from pathlib import Path
+
+import pytest
+
+jsonschema = pytest.importorskip("jsonschema")
+
+ROOT = Path(__file__).resolve().parent.parent
+SCHEMA = json.loads((ROOT / "schemas" / "approval_claim.schema.json").read_text())
+EXAMPLES = sorted((ROOT / "examples").glob("claim.*.json"))
+
+
+def test_examples_exist():
+    assert EXAMPLES, "no claim examples found to validate"
+
+
+@pytest.mark.parametrize("path", EXAMPLES, ids=lambda p: p.name)
+def test_example_matches_schema(path):
+    jsonschema.validate(json.loads(path.read_text()), SCHEMA)
+
+
+@pytest.mark.parametrize("path", EXAMPLES, ids=lambda p: p.name)
+def test_example_states_pdp_digest_explicitly(path):
+    """Absence must be a stated null, never a missing key."""
+    assert "pdp_digest" in json.loads(path.read_text())["binding"]
+
+
+def test_examples_cover_both_pdp_binding_states():
+    """An implementer must see both shapes, not infer one from the other."""
+    states = {
+        json.loads(p.read_text())["binding"]["pdp_digest"] is None for p in EXAMPLES
+    }
+    assert states == {True, False}