feat: use broker-authoritative qty in slots RPC holdings

Co-authored-by: genichin <4282227+genichin@users.noreply.github.com>

trex_core/application/account_snapshot.py

@@ -217,6 +217,19 @@ class AccountSnapshotService:
         )
         return list(snapshot.holdings)
 
+    def holdings_for_slots_rpc(self) -> tuple[list[BalanceItem], datetime]:
+        """Return freshness-validated snapshot holdings for strategy slots reads.
+
+        Unlike ``holdings_for_sell()``, this read path never performs a live broker
+        call and therefore cannot mutate broker state or mask snapshot staleness.
+        """
+        snapshot = self._load_current()
+        self._age(snapshot.holdings_captured_at, self._sell_max_age)
+        captured_at = snapshot.holdings_captured_at
+        if captured_at is None:
+            raise AccountSnapshotUnavailable("not_ready")
+        return list(snapshot.holdings), captured_at
+
     def buy_guard(
         self,
         *,