datalake: opt out of stop_translator when stopping

Previously stop_translator() seemed to allow erased translators to go
un-stopped if the datalake_manager's gate is closed. This commit adds an
early return to avoid this.

It's not clear that this is actually problematic in practice because the
notification deregistration and gate close should happen atomically in
stop(), so it seems like notifications _should_ be guaranteed to run
without thinking about the gate. But it's an easy enough check to add
in.

src/v/datalake/datalake_manager.cc

@@ -257,6 +257,10 @@ void datalake_manager::start_translator(
 }
 
 void datalake_manager::stop_translator(const model::ntp& ntp) {
+    if (_gate.is_closed()) {
+        // Cleanup should be deferred to stop().
+        return;
+    }
     auto it = _translators.find(ntp);
     if (it == _translators.end()) {
         return;