[bbr] handle role changes directly in BackboneRouter::Local (openthread#13112)

This commit updates `BackboneRouter::Local` to receive role change
events directly from the `Notifier`. Previously, `Bbr::Local` was
indirectly relying on `BackboneRouter::Leader` to emit events even
when the PBBR configuration had not changed (e.g., during role
transitions).

The previous design was fragile and created an unnecessary dependency.
`Bbr::Local` now independently tracks role changes to ensure it
correctly evaluates its own status (e.g., deciding whether to
register as the Primary BBR).

Author: abtink

src/core/backbone_router/bbr_local.cpp

@@ -240,7 +240,19 @@ void Local::SetState(State aState)
 void Local::HandleBackboneRouterPrimaryUpdate(PrimaryEvent aEvent)
 {
     OT_UNUSED_VARIABLE(aEvent);
+    UpdateState();
+}
 
+void Local::HandleNotifierEvents(Events aEvents)
+{
+    if (aEvents.Contains(kEventThreadRoleChanged))
+    {
+        UpdateState();
+    }
+}
+
+void Local::UpdateState(void)
+{
     VerifyOrExit(IsEnabled() && Get<Mle::Mle>().IsAttached());
 
     // Wait some jitter before trying to Register.

src/core/backbone_router/bbr_local.hpp

@@ -59,6 +59,7 @@
 #include "common/locator.hpp"
 #include "common/log.hpp"
 #include "common/non_copyable.hpp"
+#include "common/notifier.hpp"
 #include "common/time_ticker.hpp"
 #include "net/netif.hpp"
 #include "thread/network_data.hpp"
@@ -73,6 +74,7 @@ namespace BackboneRouter {
 class Local : public InstanceLocator, private NonCopyable
 {
     friend class ot::TimeTicker;
+    friend class ot::Notifier;
 
 public:
     typedef otBackboneRouterDomainPrefixCallback DomainPrefixCallback; ///< Domain Prefix callback.
@@ -265,6 +267,8 @@ class Local : public InstanceLocator, private NonCopyable
     };
 
     void SetState(State aState);
+    void HandleNotifierEvents(Events aEvents);
+    void UpdateState(void);
     void RemoveService(void);
     void HandleTimeTick(void);
     void AddDomainPrefixToNetworkData(void);

src/core/common/notifier.cpp

@@ -104,6 +104,9 @@ void Notifier::EmitEvents(void)
 #if (OPENTHREAD_CONFIG_THREAD_VERSION >= OT_THREAD_VERSION_1_2)
     Get<BackboneRouter::Leader>().HandleNotifierEvents(events);
 #endif
+#if OPENTHREAD_FTD && OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE
+    Get<BackboneRouter::Local>().HandleNotifierEvents(events);
+#endif
 #if OPENTHREAD_CONFIG_DHCP6_SERVER_ENABLE
     Get<Dhcp6::Server>().HandleNotifierEvents(events);
 #endif