fix(HEL-484): 中枢分时接口空 date 按当天查询

缺少或为空的 date 不再 400，按当天处理；显式历史日期保持原行为。

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: multica-agent <github@multica.ai>

tests/test_chart_data_provider.py

@@ -193,8 +193,14 @@ class DatahubChartFallbackTests(unittest.TestCase):
 
     def test_datahub_timeout_or_empty_falls_back_to_eastmoney(self):
         fallback = LookbackChartClient()
-        for hub in (FakeHub(chart=None), FakeHub(error=RuntimeError("timeout"))):
+        for hub in (
+            FakeHub(chart=None),
+            FakeHub(error=RuntimeError("timeout")),
+            FakeHub(error=RuntimeError("datahub exploded")),
+            FakeHub(chart={"points": []}),
+        ):
             EastmoneyChartClient._cache.clear()
+            fallback.requests.clear()
             client = MarketChartClient(IfindHttpClient(), fallback, hub)
             payload = client.stock_intraday("000001")
             self.assertEqual(payload["trade_date"], "2026-09-07")