style(test): 修 loader unit-sampling 测试的 ruff 告警

TC003 将 Path 导入移入 TYPE_CHECKING 块（仅注解使用），
C408 将 base = dict(...) 改为字典字面量。行为不变。

tests/unit/test_loader_unit_sampling.py

@@ -13,7 +13,7 @@ from __future__ import annotations
 
 import json
 import random
-from pathlib import Path
+from typing import TYPE_CHECKING
 
 import pytest
 
@@ -21,6 +21,9 @@ from app.harness.question_units import build_units
 from app.question_gen.loader import load_benchmark, stratified_sample
 from core.types import GeneratedQuestion
 
+if TYPE_CHECKING:
+    from pathlib import Path
+
 
 def _single(qid: str, task_type: str = "Single") -> GeneratedQuestion:
     """构造一道 single 题（无 pair 归属）。"""
@@ -38,18 +41,18 @@ def _single(qid: str, task_type: str = "Single") -> GeneratedQuestion:
 
 def _pair(pid: str, task_type: str = "AR") -> list[GeneratedQuestion]:
     """构造一个合法孪生对（original + mirror），共享 pair_id / unit_id / flip_axis。"""
-    base = dict(
-        video_id="v",
-        task_type=task_type,
-        question="?",
-        options=("A", "B", "C", "D"),
-        answer="A",
-        source_nodes=(),
-        difficulty="hard",
-        pair_id=pid,
-        unit_id=pid,
-        flip_axis="before_after",
-    )
+    base = {
+        "video_id": "v",
+        "task_type": task_type,
+        "question": "?",
+        "options": ("A", "B", "C", "D"),
+        "answer": "A",
+        "source_nodes": (),
+        "difficulty": "hard",
+        "pair_id": pid,
+        "unit_id": pid,
+        "flip_axis": "before_after",
+    }
     return [
         GeneratedQuestion(question_id=f"{pid}_o", question_role="pair_original", **base),
         GeneratedQuestion(question_id=f"{pid}_m", question_role="pair_mirror", **base),