test: 修两个 CI flaky 测试（与 #1749 无关，顺带修让 CI Gate 过）

- test_auth_bind_wechat_sync：默认昵称 '微信用户' → '小虾同学'（use case 已改，测试未同步）
- test_smart_match_noise_1743：patch datetime.now 为 NOW，避免 recency 分数随真实时间漂移导致断言失败

tests/unit/test_auth_bind_wechat_sync.py

@@ -656,7 +656,7 @@ class TestWechatSyncUseCase:
 
         assert error is None
         assert response.is_new_user is True
-        assert response.nickname == "微信用户"
+        assert response.nickname == "小虾同学"
 
     def test_username_uniqueness_suffix(self):
         """When username already exists, a numeric suffix is added."""

tests/unit/test_smart_match_noise_1743.py

@@ -84,11 +84,18 @@ class TestScoreNoiseInjection:
 
     def test_score_field_is_raw_without_noise(self):
         """r.score 始终是无噪声原始分（噪声只影响排序，不污染返回分值）。"""
+        from unittest.mock import patch
+        from datetime import datetime as _dt
         assets = _make_tied_assets(5)
         raw_scores = {score_asset(a, now=NOW)[0] for a in assets}
-        results = smart_select_assets(assets, rng=random.Random(42))
+        # 冻结 datetime.now，与 score_asset 用的 NOW 一致（recency 时变分数需稳定）
+        with patch("packages.domain.smart_match.datetime") as mock_dt:
+            mock_dt.now.return_value = NOW
+            # 保留 datetime 构造函数行为（如 datetime(...) 调用）
+            mock_dt.side_effect = _dt
+            results = smart_select_assets(assets, rng=random.Random(42))
         for r in results:
-            assert r.score in raw_scores
+            assert r.score in raw_scores, f"score={r.score} 不在 raw_scores={raw_scores}"
 
     def test_rng_deterministic_same_seed(self):
         """同一种子多次调用结果完全一致（可复现，测试可依赖）。"""