[netdata] enhance FindContext() methods and Lowpan::Context (#11836)

This commit enhances the `FindContext()` methods and converts the
`Lowpan::Context` struct into a class.

The `FindContext()` methods are updated as follows:
- Renamed to `FindContextForAddress()` and `FindContextForId()` to
  more accurately reflect their function.
- The return type is changed from `Error` to `void`. Success is now
  indicated by checking the `IsValid()` state of the output `Context`
  object (matching how `Lowpan` class uses the `Context`). This
  change simplifies the callers and harmonizes the context check
  across different modules.

The `Lowpan::Context` struct is converted into a class, encapsulating
its members by making them private and introducing public getters.

src/core/thread/mle_ftd.cpp

@@ -1910,14 +1910,16 @@ Error Mle::ProcessAddressRegistrationTlv(RxInfo &aRxInfo, Child &aChild)
             IgnoreError(aRxInfo.mMessage.Read(offsetRange, address.GetIid()));
             offsetRange.AdvanceOffset(sizeof(Ip6::InterfaceIdentifier));
 
-            if (Get<NetworkData::Leader>().GetContext(contextId, context) != kErrorNone)
+            Get<NetworkData::Leader>().FindContextForId(contextId, context);
+
+            if (!context.IsValid())
             {
                 LogWarn("Failed to get context %u for compressed address from child 0x%04x", contextId,
                         aChild.GetRloc16());
                 continue;
             }
 
-            address.SetPrefix(context.mPrefix);
+            address.SetPrefix(context.GetPrefix());
         }
         else
         {