[dns] update Name::ValidateName() for max-length names (#12127)

`ValidateName()` did not correctly handle names that were exactly the
maximum allowed length (`kMaxNameLength`). A name of this length is
only valid if it ends with a trailing dot. Otherwise, when encoded,
the added root label causes the encoded name to exceed the
`kMaxEncodedLength` of 255 bytes.

This commit updates `ValidateName()` to enforce that any name with
length equal to `kMaxNameLength` must end with a dot character.

It also updates the `TestDnsName` unit test to verify this corrected
behavior, ensuring `ValidateName()` and `AppendName()` handle such
names consistently.

src/core/net/dns_types.cpp

@@ -775,6 +775,15 @@ Error Name::ValidateName(const char *aName)
     VerifyOrExit(length > 0, error = kErrorInvalidArgs);
     VerifyOrExit(length <= kMaxNameLength, error = kErrorInvalidArgs);
 
+    if (length == kMaxNameLength)
+    {
+        // Allow `kMaxNameLength` only if the `aName` ends with a dot.
+        // This ensures that the encoded name always fits within the
+        // `kMaxEncodedLength = 255` octets.
+
+        VerifyOrExit(aName[length - 1] == kLabelSeparatorChar, error = kErrorInvalidArgs);
+    }
+
     do
     {
         ch = aName[index];

tests/unit/test_dns.cpp

@@ -485,9 +485,13 @@ void TestDnsName(void)
 
         IgnoreError(message->SetLength(0));
 
-        printf("\"%s\"\n", maxLengthName);
+        printf("\"%s\" (len:%u)\n", maxLengthName, static_cast<uint16_t>(strlen(maxLengthName)));
+
+        SuccessOrQuit(Dns::Name::ValidateName(maxLengthName));
 
         SuccessOrQuit(Dns::Name::AppendName(maxLengthName, *message));
+
+        VerifyOrQuit(message->GetLength() == Dns::Name::kMaxNameSize);
     }
 
     printf("----------------------------------------------------------------\n");
@@ -497,7 +501,9 @@ void TestDnsName(void)
     {
         IgnoreError(message->SetLength(0));
 
-        printf("\"%s\"\n", invalidName);
+        printf("\"%s\" (len:%u)\n", invalidName, static_cast<uint16_t>(strlen(invalidName)));
+
+        VerifyOrQuit(Dns::Name::ValidateName(invalidName) != kErrorNone);
 
         VerifyOrQuit(Dns::Name::AppendName(invalidName, *message) == kErrorInvalidArgs);
     }