[dns-client] fix finalizing query from HandleTimer() (#8982)

This commit fixes an issue in `HandleTimer()` where we need to use
`break` instead of `continue` after calling `FinalizeQuery()`. Note
that we now have two loops, an outer loop on `mMainQueries` and then
an inner loop on queries associated with a `mainQuery`. When we
finalize a query, its related `mainQuery` (along with all its
sub-queries) are finalized and removed.

src/core/net/dns_client.cpp

@@ -1450,7 +1450,7 @@ void Client::HandleTimer(void)
                 if (info.mTransmissionCount >= info.mConfig.GetMaxTxAttempts())
                 {
                     FinalizeQuery(*query, kErrorResponseTimeout);
-                    continue;
+                    break;
                 }
 
                 IgnoreError(SendQuery(*query, info, /* aUpdateTimer */ false));