fix: normalize snapshot read failures to unavailable in slots RPC path

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

tests/application/test_account_snapshot.py

@@ -36,6 +36,11 @@ class FakeAccountBroker:
         return self.cash
 
 
+class BrokenLoadStore(InMemoryAccountSnapshotStore):
+    def load(self, account_hash: str) -> AccountSnapshot | None:
+        raise RuntimeError("db read failed")
+
+
 def _wire():
     now = [datetime(2026, 7, 29, tzinfo=UTC)]
     broker = FakeAccountBroker()
@@ -177,3 +182,19 @@ def test_slots_rpc_uses_snapshot_only_with_sell_ttl_freshness():
     with pytest.raises(AccountSnapshotUnavailable) as raised:
         service.holdings_for_slots_rpc()
     assert raised.value.code == "expired"
+
+
+def test_slots_rpc_normalizes_snapshot_store_read_failure():
+    broker = FakeAccountBroker()
+    service = AccountSnapshotService(
+        broker=broker,
+        store=BrokenLoadStore(),
+        buy_max_age_sec=600,
+        sell_max_age_sec=3600,
+        clock=lambda: datetime(2026, 7, 29, tzinfo=UTC),
+        rng=lambda: 0.5,
+    )
+
+    with pytest.raises(AccountSnapshotUnavailable) as raised:
+        service.holdings_for_slots_rpc()
+    assert raised.value.code == "read_failed"