[lowpan] check datagramSize in Decompress and DecompressUdpHeader (#13276)

This commit adds robust bounds validation checks inside the lowpan
decompression routines to prevent potential integer underflows.

Specifically:
- In Lowpan::Decompress, we ensure that the declared datagramSize is
  large enough to contain the IPv6 header being decompressed at the
  current offset. This prevents underflow during recursive calls
  (e.g., for tunneled packets) when the datagram size is smaller
  than the combined headers.
- In Lowpan::DecompressUdpHeader, we verify that the remaining
  datagram length is sufficient to fit a UDP header, preventing
  underflow when calculating the UDP length from the offset.

src/core/thread/lowpan.cpp

@@ -970,6 +970,7 @@ Error Lowpan::DecompressUdpHeader(Message &aMessage, FrameData &aFrameData, uint
     }
     else
     {
+        VerifyOrExit(aDatagramLength >= aMessage.GetOffset() + sizeof(Ip6::UdpHeader), error = kErrorParse);
         udpHeader.SetLength(aDatagramLength - aMessage.GetOffset());
     }
 
@@ -1035,6 +1036,7 @@ Error Lowpan::Decompress(Message              &aMessage,
 
     if (aDatagramLength)
     {
+        VerifyOrExit(aDatagramLength >= currentOffset + sizeof(Ip6::Header), error = kErrorParse);
         ip6PayloadLength = BigEndian::HostSwap16(aDatagramLength - currentOffset - sizeof(Ip6::Header));
     }
     else