[border-agent] introduce BorderAgent namespace (#12002)

This change reorganizes the `BorderAgent` related classes into a new
`MeshCoP::BorderAgent` namespace to improve code structure and clarity.

The following changes are included:

- `MeshCoP::BorderAgent` class is renamed to `Manager` and placed
  within the new `MeshCoP::BorderAgent` namespace.
- `MeshCoP::BorderAgentTracker` is renamed to `Tracker` under the
   new namespace.
- `EphemeralKeyManager` is moved from being a nested class in
  `BorderAgent` to `MeshCoP::BorderAgent::EphemeralKeyManager`.
- `EphemeralKeyManager` is now a direct member of the `Instance` class,
  simplifying accessing it.

All related calls and test files are updated to reflect these
changes.

src/core/common/notifier.cpp

@@ -131,7 +131,7 @@ void Notifier::EmitEvents(void)
     Get<AnnounceSender>().HandleNotifierEvents(events);
 #endif
 #if OPENTHREAD_CONFIG_BORDER_AGENT_ENABLE
-    Get<MeshCoP::BorderAgent>().HandleNotifierEvents(events);
+    Get<MeshCoP::BorderAgent::Manager>().HandleNotifierEvents(events);
 #endif
 #if OPENTHREAD_CONFIG_MLR_ENABLE || (OPENTHREAD_FTD && OPENTHREAD_CONFIG_TMF_PROXY_MLR_ENABLE)
     Get<MlrManager>().HandleNotifierEvents(events);