feat: merge datetime fields in DataFrame output, hide MinuteBar internal fields

- K-line: daily+ periods output 'date' only, minute periods output 'datetime'
- Transactions (tick-by-tick): combine date param + hour/minute into 'datetime'
- XdxrRecord, HistoricalFundFlow: year/month/day merged to 'date'
- MinuteBar: rename unknown_1 to _unknown_1 (hidden from DataFrame)
- MinuteBar: add datetime column computed from bar index (A-share 240-bar pattern)
- get_minute_time_data: use history endpoint only (current-day endpoint broken in pytdx too)
- Update all examples to reflect new DataFrame column names

examples/01_connection/async_connect.py

@@ -1,7 +1,8 @@
 """演示：异步客户端连接与基本用法。"""
 
 import asyncio
-from easy_tdx import AsyncTdxClient, Market, KlineCategory
+
+from easy_tdx import AsyncTdxClient, KlineCategory, Market
 
 
 async def main():
@@ -12,13 +13,8 @@ async def main():
 
     # 自动优选服务器
     async with AsyncTdxClient.from_best_host() as c:
-        bars = await c.get_security_bars(
-            Market.SH, "600000", KlineCategory.DAY, 0, 5
-        )
-        for bar in bars:
-            print(f"{bar.year}-{bar.month:02d}-{bar.day:02d} "
-                  f"开:{bar.open:.2f} 高:{bar.high:.2f} "
-                  f"低:{bar.low:.2f} 收:{bar.close:.2f}")
+        df = await c.get_security_bars(Market.SH, "600000", KlineCategory.DAY, 0, 5)
+        print(df.to_string(index=False))
 
 
 asyncio.run(main())