Enhance DaliGatewayCache: add support for multiple status update callbacks

Signed-off-by: Tony <tonylu@tony-cloud.com>

tests/dali_gateway_test.cpp

@@ -30,15 +30,19 @@ int main() {
   });
 
   int status_updates = 0;
+  int observer_updates = 0;
   DaliGatewayStatusUpdate last_update;
   cache.setStatusUpdateCallback([&](const DaliGatewayStatusUpdate& update) {
     ++status_updates;
     last_update = update;
   });
+  cache.addStatusUpdateCallback(
+      [&](const DaliGatewayStatusUpdate&) { ++observer_updates; });
 
   assert(cache.setGroupMask(7, 4, 0x0001));
   assert(cache.mirrorForwardFrame(7, 0x80, 42));
   assert(status_updates == 1);
+  assert(observer_updates == status_updates);
   assert(last_update.channel == 7);
   assert(last_update.target.kind == DaliGatewayTargetKind::group);
   assert(last_update.target.value == 0);
@@ -47,6 +51,12 @@ int main() {
   assert(cache.groupStatus(7, 0).actualLevel == 42);
   assert(cache.addressState(7, 4).status.actualLevel == 42);
 
+  // Replacing the owning callback must not remove additive adapter observers.
+  cache.setStatusUpdateCallback({});
+  assert(cache.setActualLevel(7, 4, 43));
+  assert(status_updates == 1);
+  assert(observer_updates == 2);
+
   // The command-state cache tracks DTR-dependent settings and applies a
   // broadcast command to every state known by the adapter.
   cache.mirrorForwardFrame(7, 0xA3, 12);