chore(api): canon log follow-ups — hardening, unified 422, contract doc

- canon_log.py: stringify jsonschema error-path segments before sorting so a
  future schema mixing array indices and object keys can't TypeError (#1)
- main.py: RequestValidationError handler reshapes pydantic body failures into
  the same {"detail": {"canon_log_errors": [...]}} envelope as schema failures,
  so the client parses ONE 422 shape (#2a) + tests
- pytest.ini: filter the starlette/httpx TestClient deprecation → pristine output (#4)
- docs/canon-log.md: living contract for the client (Plan B) — schemas, the §7
  boundary, the unified error envelope, per-role injection (#3)

Full suite 26 passing, 0 warnings.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

api/tests/test_endpoints.py

@@ -36,3 +36,12 @@ def test_every_role_endpoint_rejects_an_invalid_canon_log():
 def test_missing_canon_log_is_a_422():
     r = client.post("/dm/narrate", json={})
     assert r.status_code == 422
+    assert "canon_log_errors" in r.json()["detail"]
+
+
+def test_malformed_body_uses_the_unified_error_shape():
+    # A pydantic body failure returns the SAME envelope as a schema failure,
+    # so the client only ever parses one 422 shape.
+    r = client.post("/dm/narrate", json={"canon_log": "not-a-dict"})
+    assert r.status_code == 422
+    assert "canon_log_errors" in r.json()["detail"]