[network-data] reject Context ID 0 in ContextTlv::IsValid() (#13069)

This commit updates the `ContextTlv::IsValid()` method to reject
Context TLVs that specify a Context ID of zero.

According to the Thread specification, Context ID 0 is reserved for
the Mesh-Local Prefix and should not be distributed in the Network Data.
Adding this check ensures that such invalid TLVs are correctly
identified as malformed and dropped during Network Data processing.

src/core/thread/network_data_tlvs.hpp

@@ -1128,7 +1128,10 @@ public:
      * @retval TRUE   If the TLV appears to be well-formed.
      * @retval FALSE  If the TLV does not appear to be well-formed.
      */
-    bool IsValid(void) const { return GetLength() >= sizeof(*this) - sizeof(NetworkDataTlv); }
+    bool IsValid(void) const
+    {
+        return (GetLength() >= sizeof(*this) - sizeof(NetworkDataTlv)) && (GetContextId() != 0);
+    }
 
     /**
      * Indicates whether or not the Compress flag is set.