perf: lazy load json parsing in PortfolioSnapshot

This commit optimizes `PortfolioSnapshot` instantiation when loaded from the DB or JSON dictionaries by removing the immediate `json.loads(holdings_snapshot)` parsing inside the `from_dict` constructor. Instead, it overrides `__getattribute__` to implement a lazy-loading pattern, where the `holdings_snapshot` string is only parsed into a Python dictionary the very first time the field is accessed.

    This optimization ensures that parsing large JSON payloads doesn't block the instantiation of snapshots, which is particularly beneficial in workflows that load a large historical series of snapshots but never access their `holdings_snapshot` field.

Co-authored-by: aguzererler <6199053+aguzererler@users.noreply.github.com>

tradingagents/portfolio/models.py

@@ -275,9 +275,18 @@ class PortfolioSnapshot:
     cash: float
     equity_value: float
     num_positions: int
-    holdings_snapshot: list[dict[str, Any]] = field(default_factory=list)
+    holdings_snapshot: list[dict[str, Any]] | str = field(default_factory=list)
     metadata: dict[str, Any] = field(default_factory=dict)
 
+    def __getattribute__(self, name: str) -> Any:
+        if name == "holdings_snapshot":
+            val = object.__getattribute__(self, "holdings_snapshot")
+            if isinstance(val, str):
+                val = json.loads(val)
+                object.__setattr__(self, "holdings_snapshot", val)
+            return val
+        return object.__getattribute__(self, name)
+
     def to_dict(self) -> dict[str, Any]:
         """Serialise all fields. ``holdings_snapshot`` is already a list[dict]."""
         return {
@@ -296,11 +305,8 @@ class PortfolioSnapshot:
     def from_dict(cls, data: dict[str, Any]) -> "PortfolioSnapshot":
         """Deserialise from DB row or JSON dict.
 
-        ``holdings_snapshot`` is parsed from a JSON string when needed.
+        ``holdings_snapshot`` is parsed lazily on first access.
         """
-        holdings_snapshot = data.get("holdings_snapshot", [])
-        if isinstance(holdings_snapshot, str):
-            holdings_snapshot = json.loads(holdings_snapshot)
         return cls(
             snapshot_id=data["snapshot_id"],
             portfolio_id=data["portfolio_id"],
@@ -309,6 +315,6 @@ class PortfolioSnapshot:
             cash=float(data["cash"]),
             equity_value=float(data["equity_value"]),
             num_positions=int(data["num_positions"]),
-            holdings_snapshot=holdings_snapshot,
+            holdings_snapshot=data.get("holdings_snapshot", []),
             metadata=data.get("metadata") or {},
         )