fix(health): preserve transient failure codes through stale snapshot fallback

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

trex_core/application/account_snapshot.py

@@ -214,6 +214,7 @@ class AccountSnapshotService:
 
     def holdings_for_sell(self) -> list[BalanceItem]:
         """Prefer a live balance; fallback only on transient transport failure."""
+        transient_code: str | None = None
         try:
             return self._broker.get_balance()
         except BrokerBalanceError as exc:
@@ -221,8 +222,14 @@ class AccountSnapshotService:
                 raise AccountSnapshotUnavailable(
                     "live_read_failed", detail_code=exc.code
                 ) from None
+            transient_code = exc.code
         snapshot = self._load_current()
-        age = self._age(snapshot.holdings_captured_at, self._sell_max_age)
+        try:
+            age = self._age(snapshot.holdings_captured_at, self._sell_max_age)
+        except AccountSnapshotUnavailable as exc:
+            raise AccountSnapshotUnavailable(
+                exc.code, detail_code=transient_code
+            ) from None
         _log.warning(
             "sell balance using account snapshot: age_sec=%.0f",
             age,