fix(question_gen): raise ValueError on UPDATE of missing rows

record_run_end, update_gates, and update_difficulty now check
cursor.rowcount after UPDATE+commit and raise ValueError if 0 rows
were affected. Prevents silent telemetry loss.

Adds three negative-path tests:
- test_record_run_end_missing_run_raises
- test_update_gates_missing_item_raises
- test_update_difficulty_missing_item_raises

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/unit/test_run_store.py

@@ -242,3 +242,27 @@ class TestQuestionGenStore:
         """查询不存在的 run_id 应报错。"""
         with pytest.raises(ValueError, match="run_id"):
             store.get_run_stats("nonexistent-run")
+
+    def test_record_run_end_missing_run_raises(self, store: QuestionGenStore) -> None:
+        """对不存在的 run_id 调用 record_run_end 应报错。"""
+        stats = RunStats(total_slots=10, accepted=5, rejected=3, heavy_sampled=2)
+        with pytest.raises(ValueError, match="run_id"):
+            store.record_run_end(
+                run_id="ghost-run", status="completed", stats=stats
+            )
+
+    def test_update_gates_missing_item_raises(self, store: QuestionGenStore) -> None:
+        """对不存在的 item_id 调用 update_gates 应报错。"""
+        report = _MockGateReport(
+            key_verify=_GateResult(_Verdict.PASS, "ok"),
+            blind_answer=_GateResult(_Verdict.PASS, "ok"),
+            multi_true=_GateResult(_Verdict.PASS, "ok"),
+            leak_test=_GateResult(_Verdict.PASS, "ok"),
+        )
+        with pytest.raises(ValueError, match="item_id"):
+            store.update_gates(item_id="ghost-item", report=report)
+
+    def test_update_difficulty_missing_item_raises(self, store: QuestionGenStore) -> None:
+        """对不存在的 item_id 调用 update_difficulty 应报错。"""
+        with pytest.raises(ValueError, match="item_id"):
+            store.update_difficulty(item_id="ghost-item", difficulty_steps=5)