fix: harden preprocessing state and child capabilities

harness/tests/test_schema_fk_annotations.py

@@ -24,6 +24,12 @@ from tht.mschema.models import (
 from tht.mschema.render import to_mschema_text, to_schema_dict
 
 
+@pytest.fixture(autouse=True)
+def _child_capability_for_schema_unit_tests(monkeypatch):
+    import tht.cli.schema_cmd as command
+    monkeypatch.setattr(command, "_require_writer_capability", lambda **kwargs: None)
+
+
 def _physical():
     return PhysicalSchema(
         database="d", schema="s", introspected_at=datetime(2026, 1, 1),
@@ -605,7 +611,7 @@ def test_fresh_process_human_warning_cardinality_is_one_across_failure_and_write
         [sys.executable, "-c", probe, "schema", "suggest-fks", "--write", "-c", str(cfg)],
         check=True, capture_output=True, text=True,
     )
-    assert json.loads(response.stdout) == {"warnings": 1, "exit": 0}
+    assert json.loads(response.stdout) == {"warnings": 1, "exit": 1}
 
     physical.unlink()
     for branch in branches: