multi: enforce strict TLV length checks

This commit improves TLV decoding safety and consistency across multiple
packages by enforcing fixed-length requirements and adding unit tests to
prevent malformed TLV records from being accepted.

Changes include:

- lnwire:
  * Enforce 8-byte length in Fee TLV decoder.
  * Enforce PubNonceSize in Musig2Nonce TLV decoder.
  * Enforce 8-byte length in ShortChannelID TLV decoder.
  * Added roundtrip and invalid length tests for Fee, Musig2Nonce,
    and ShortChannelID records.

- routing/route:
  * Enforce Vertex TLV length (33 bytes).
  * Added encode/decode and invalid length tests for Vertex.

- tlv:
  * Enforce correct length in DBytes33 decoder (33 bytes).
  * Added tests ensuring all fixed-size primitive decoders reject
    incorrect TLV lengths.

By strictly validating TLV lengths, we prevent malformed or corrupted
TLV records from being silently accepted, improving protocol safety.

routing/route/route.go

@@ -112,7 +112,7 @@ func encodeVertex(w io.Writer, val interface{}, _ *[8]byte) error {
 }
 
 func decodeVertex(r io.Reader, val interface{}, _ *[8]byte, l uint64) error {
-	if b, ok := val.(*Vertex); ok {
+	if b, ok := val.(*Vertex); ok && l == VertexSize {
 		_, err := io.ReadFull(r, b[:])
 		return err
 	}