refactor(useragent): using request/response for correct multiplexing behaviour

useragent/lib/providers/vault_state.dart

@@ -13,9 +13,7 @@ Future<VaultState?> vaultState(Ref ref) async {
     return null;
   }
 
-  await conn.send(UserAgentRequest(queryVaultState: Empty()));
-
-  final resp = await conn.receive();
+  final resp = await conn.request(UserAgentRequest(queryVaultState: Empty()));
   if (resp.whichPayload() != UserAgentResponse_Payload.vaultState) {
     talker.warning('Expected vault state response, got ${resp.whichPayload()}');
     return null;

useragent/lib/providers/vault_state.g.dart

@@ -46,4 +46,4 @@ final class VaultStateProvider
   }
 }
 
-String _$vaultStateHash() => r'1fd975a9661de1f62beef9eb1c7c439f377a8b88';
+String _$vaultStateHash() => r'97085e49bc3a296e36fa6c04a8f4c9abafac0835';