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>

app/question_gen/run_store.py

@@ -212,7 +212,7 @@ class QuestionGenStore:
             批次统计摘要。
         """
         now = datetime.now(tz=UTC).isoformat(timespec="seconds")
-        self._conn.execute(
+        cursor = self._conn.execute(
             """
             UPDATE question_gen_runs
             SET ended_at=?, status=?, total_slots=?, accepted=?, rejected=?, heavy_sampled=?
@@ -229,6 +229,8 @@ class QuestionGenStore:
             ),
         )
         self._conn.commit()
+        if cursor.rowcount == 0:
+            raise ValueError(f"run_id 不存在: {run_id}")
         logger.info(
             "出题批次已结束: run_id={}, status={}, accepted={}/{}",
             run_id,
@@ -307,7 +309,7 @@ class QuestionGenStore:
             属性，每个属性具有 .verdict.value 和 .reason；以及 passed/reject_reason 属性）。
         """
         final_status = "accepted" if report.passed else "rejected"
-        self._conn.execute(
+        cursor = self._conn.execute(
             """
             UPDATE question_gen_items
             SET gate_key_verify=?, gate_blind_answer=?, gate_multi_true=?,
@@ -325,6 +327,8 @@ class QuestionGenStore:
             ),
         )
         self._conn.commit()
+        if cursor.rowcount == 0:
+            raise ValueError(f"item_id 不存在: {item_id}")
 
     def update_difficulty(self, item_id: str, difficulty_steps: int) -> None:
         """更新重量抽检产出的 Agent 步数。
@@ -336,11 +340,13 @@ class QuestionGenStore:
         difficulty_steps : int
             Agent 完成该题所需步数。
         """
-        self._conn.execute(
+        cursor = self._conn.execute(
             "UPDATE question_gen_items SET difficulty_steps=? WHERE item_id=?",
             (difficulty_steps, item_id),
         )
         self._conn.commit()
+        if cursor.rowcount == 0:
+            raise ValueError(f"item_id 不存在: {item_id}")
 
     def get_run_stats(self, run_id: str) -> RunStats:
         """查询批次统计摘要。