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/03_kline/security_bars.py

@@ -5,19 +5,9 @@ K 线类别:
   KlineCategory.DAY    / WEEK   / MONTH  / YEAR
 """
 
-import pandas as pd
-from easy_tdx import TdxClient, Market, KlineCategory
+from easy_tdx import KlineCategory, Market, TdxClient
 
 with TdxClient.from_best_host() as c:
-    bars = c.get_security_bars(Market.SZ, "002176", KlineCategory.DAY, 0, 100)
-    df = pd.DataFrame([{
-        "日期": f"{b.year}-{b.month:02d}-{b.day:02d}",
-        "开盘": b.open,
-        "最高": b.high,
-        "最低": b.low,
-        "收盘": b.close,
-        "成交量": b.vol,
-        "成交额": b.amount,
-    } for b in reversed(bars)])
-    print("上证指数 日K线:")
+    df = c.get_security_bars(Market.SZ, "002176", KlineCategory.DAY, 0, 100)
+    print("江特电机 日K线:")
     print(df.to_string(index=False))