fix(HEL-412): 刷新降级不再整次失败，并补齐准备中提示

手动刷新与自动补跑共用可用数据判定：日线推算或上一交易日快照记为部分/准备中成功，避免前端误报刷新失败。HTTP JSON 解析错误不再把请求正文写入日志。

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

tests/test_datahub_bridge.py

@@ -198,7 +198,8 @@ class DatahubBridgeTests(unittest.TestCase):
         self.assertEqual(canonical["vol"], 100000.0)
         self.assertEqual(canonical["amount"], 2000000.0)
 
-    def test_heaven_keeps_legacy_even_when_read_flag_is_on(self) -> None:
+    def test_heaven_keeps_legacy_on_first_batch_even_when_read_flag_is_on(self) -> None:
+        """问天未永久冻结；首批只读接入仍走旧链路，后续迁移可以纳入。"""
         self.assertTrue(looks_like_heaven("backend.features.heaven.market_context", "backend/features/heaven/market_context.py"))
         self.assertFalse(looks_like_heaven("backend.features.market.service", "backend/features/market/service.py"))
         client = FakeClient()