feat(store): set_schema_linking validates then writes the F4 artifact

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

harness/tests/test_set_schema_linking.py

@@ -0,0 +1,35 @@
+"""L1: store.set_schema_linking — validate against SchemaLinking, then write."""
+import json
+
+import pytest
+from pydantic import ValidationError
+
+from tht.config import DatabaseConfig
+from tht.session.models import SchemaLinking
+from tht.session.store import create_session, set_schema_linking
+
+
+def _db():
+    return DatabaseConfig(database="testdb", user="u", password="p", **{"schema": "public"})  # noqa: S106
+
+
+def test_writes_and_revalidates(tmp_path):
+    m = create_session("q", _db(), tmp_path)
+    data = {
+        "question": "q riscritta",
+        "candidates": [{"kind": "table", "name": "fact_x", "decision": "promoted"}],
+        "joins": [{"from": "a.k", "to": "b.k"}],
+    }
+    path = set_schema_linking(m.id, data, tmp_path)
+    assert path.exists()
+    reloaded = json.loads(path.read_text())
+    assert reloaded["candidates"][0]["name"] == "fact_x"
+    assert reloaded["joins"][0]["from"] == "a.k"  # 'from' alias round-trips
+    SchemaLinking.model_validate(reloaded)  # re-validates clean
+
+
+def test_rejects_invalid_and_writes_nothing(tmp_path):
+    m = create_session("q", _db(), tmp_path)
+    with pytest.raises(ValidationError):
+        set_schema_linking(m.id, {"question": "q", "bogus": 1}, tmp_path)  # extra=forbid
+    assert not (tmp_path / m.id / "schema_linking.json").exists()