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
 	}

routing/route/route_test.go

@@ -8,6 +8,7 @@ import (
 	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/record"
+	"github.com/lightningnetwork/lnd/tlv"
 	"github.com/stretchr/testify/require"
 )
 
@@ -430,3 +431,57 @@ func TestBlindedHopFee(t *testing.T) {
 	require.Equal(t, lnwire.MilliSatoshi(0), route.HopFee(3))
 	require.Equal(t, lnwire.MilliSatoshi(0), route.HopFee(4))
 }
+
+// makeVertex creates a test Vertex with sequential byte values for testing
+// TLV encoding/decoding.
+func makeVertex() Vertex {
+	var v Vertex
+	for i := range v {
+		v[i] = byte(i)
+	}
+
+	return v
+}
+
+// TestVertexTLVEncodeDecode tests that we're able to properly encode and decode
+// Vertex within TLV streams.
+func TestVertexTLVEncodeDecode(t *testing.T) {
+	t.Parallel()
+
+	vertex := makeVertex()
+
+	var extraData lnwire.ExtraOpaqueData
+	require.NoError(t, extraData.PackRecords(&vertex))
+
+	var vertex2 Vertex
+	tlvs, err := extraData.ExtractRecords(&vertex2)
+	require.NoError(t, err)
+
+	require.Contains(t, tlvs, tlv.Type(0))
+	require.Equal(t, vertex, vertex2)
+}
+
+// TestVertexTypeDecodeInvalidLength ensures that decoding a Vertex TLV
+// with an invalid length (anything other than 33) fails with an error.
+func TestVertexTypeDecodeInvalidLength(t *testing.T) {
+	t.Parallel()
+
+	vertex := makeVertex()
+
+	var extraData lnwire.ExtraOpaqueData
+	require.NoError(t, extraData.PackRecords(&vertex))
+
+	// Corrupt the TLV length field to simulate malformed input.
+	// Byte 1 contains the varint size encoding. Since 33 bytes fits into
+	// a single varint byte, we can directly modify extraData[1].
+	extraData[1] = VertexSize + 1
+
+	var out Vertex
+	_, err := extraData.ExtractRecords(&out)
+	require.Error(t, err)
+
+	extraData[1] = VertexSize - 1
+
+	_, err = extraData.ExtractRecords(&out)
+	require.Error(t, err)
+}