fix(dataflows): make the Yahoo news window UTC and end-exclusive

- the upper bound was inclusive, so an article stamped exactly midnight after
  end_date leaked into a historical run
- flat epoch timestamps were parsed in host-local time and offset-aware stamps had
  tzinfo stripped without converting, making filtering machine-dependent
- normalize every operand to UTC and use a half-open [start, end + 1 day) #1126

tradingagents/dataflows/yfinance_news.py

@@ -1,7 +1,7 @@
 """yfinance-based news data fetching functions."""
 
 import contextlib
-from datetime import datetime
+from datetime import datetime, timedelta, timezone
 
 import yfinance as yf
 from dateutil.relativedelta import relativedelta
@@ -11,6 +11,16 @@ from .stockstats_utils import yf_retry
 from .symbol_utils import normalize_symbol
 
 
+def _as_utc(dt: datetime) -> datetime:
+    """Normalize a datetime to UTC-aware; a naive value is assumed to be UTC.
+
+    Window bounds arrive naive (parsed from ``yyyy-mm-dd``) while article
+    timestamps may be offset-aware, so every operand is normalized before
+    comparison. Without this the filter depends on the host timezone (#1126).
+    """
+    return dt.replace(tzinfo=timezone.utc) if dt.tzinfo is None else dt.astimezone(timezone.utc)
+
+
 def _extract_article_data(article: dict) -> dict:
     """Extract article data from yfinance news format (handles nested 'content' structure)."""
     # Handle nested content structure
@@ -46,8 +56,10 @@ def _extract_article_data(article: dict) -> dict:
         pub_date = None
         ts = article.get("providerPublishTime")
         if ts:
+            # Epoch seconds are UTC; parse them as UTC-aware so filtering does
+            # not shift with the host timezone (#1126).
             with contextlib.suppress(ValueError, OSError, TypeError):
-                pub_date = datetime.fromtimestamp(ts)
+                pub_date = datetime.fromtimestamp(ts, tz=timezone.utc)
         return {
             "title": article.get("title", "No title"),
             "summary": article.get("summary", ""),
@@ -58,17 +70,18 @@ def _extract_article_data(article: dict) -> dict:
 
 
 def _in_news_window(pub_date, start_dt, end_dt) -> bool:
-    """Whether an article belongs in the [start_dt, end_dt] window.
+    """Whether an article belongs in the half-open window ``[start, end + 1 day)``.
 
-    Dated articles are kept only if they fall in the window. An undated article
-    is kept only when the window reaches the present (live run) — in a
-    historical/backtest window it's excluded, since we can't prove it isn't
-    future news (look-ahead safety, #992/#1007).
+    Every operand is normalized to UTC, and the upper bound is exclusive so an
+    article stamped exactly at midnight after ``end_dt`` cannot leak into a
+    historical run (#1126). An undated article is kept only when the window
+    reaches the present (live run) — in a historical/backtest window it's
+    excluded, since we can't prove it isn't future news (#992/#1007).
     """
+    end = _as_utc(end_dt)
     if pub_date is not None:
-        naive = pub_date.replace(tzinfo=None) if hasattr(pub_date, "replace") else pub_date
-        return start_dt <= naive <= end_dt + relativedelta(days=1)
-    return end_dt >= datetime.now() - relativedelta(days=1)
+        return _as_utc(start_dt) <= _as_utc(pub_date) < end + timedelta(days=1)
+    return end >= datetime.now(timezone.utc) - timedelta(days=1)
 
 
 def get_news_yfinance(