[message] introduce ReadAndAdvance() to simplify sequential parsing (#13106)

This commit introduces `Message::ReadAndAdvance()` and its template
flavor to the `Message` class. This helper method reads data from a
`Message` at a given `OffsetRange` and advances the `OffsetRange` by
the number of bytes read upon success.

Sequential parsing of structured data (such as TLVs or protocol
headers) is a common pattern across the OpenThread codebase.
Previously, this required two separate calls: one to `Read()` and
another to `AdvanceOffset()`. The new `ReadAndAdvance()` method
consolidates these into a single, safer operation that ensures the
offset is only advanced if the read operation succeeds.

This commit updates numerous call sites across the core stack
(MLE, BBR, DatasetManager, NetworkDiagnostic, DHCPv6, etc.) to use
the new helper, improving code clarity and reducing boilerplate.

tests/unit/test_dhcp6_pd_client.cpp

@@ -342,10 +342,9 @@ void Dhcp6Msg::ParseFrom(const Message &aMessage)
     Clear();
     offsetRange.InitFromMessageFullLength(aMessage);
 
-    SuccessOrQuit(aMessage.Read(offsetRange, header));
+    SuccessOrQuit(aMessage.ReadAndAdvance(offsetRange, header));
     mMsgType       = header.GetMsgType();
     mTransactionId = header.GetTransactionId();
-    offsetRange.AdvanceOffset(sizeof(header));
 
     while (!offsetRange.IsEmpty())
     {
@@ -432,8 +431,7 @@ void Dhcp6Msg::ParseFrom(const Message &aMessage)
             break;
 
         case Dhcp6::Option::kIaPd:
-            SuccessOrQuit(aMessage.Read(optionOffsetRange, iaPdOption));
-            optionOffsetRange.AdvanceOffset(sizeof(iaPdOption));
+            SuccessOrQuit(aMessage.ReadAndAdvance(optionOffsetRange, iaPdOption));
             VerifyOrQuit(!mIaPds.ContainsMatching(iaPdOption.GetIaid()));
 
             iaPd = mIaPds.PushBack();