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.

lnwire/musig2_test.go

@@ -0,0 +1,62 @@
+package lnwire
+
+import (
+	"testing"
+
+	"github.com/btcsuite/btcd/btcec/v2/schnorr/musig2"
+	"github.com/stretchr/testify/require"
+)
+
+// makeNonce creates a test Musig2Nonce with sequential byte values for testing
+// TLV encoding/decoding.
+func makeNonce() Musig2Nonce {
+	var n Musig2Nonce
+	for i := range n {
+		n[i] = byte(i)
+	}
+
+	return n
+}
+
+// TestMusig2NonceEncodeDecode tests that we're able to properly encode and
+// decode Musig2Nonce within TLV streams.
+func TestMusig2NonceEncodeDecode(t *testing.T) {
+	t.Parallel()
+
+	nonce := makeNonce()
+
+	var extraData ExtraOpaqueData
+	require.NoError(t, extraData.PackRecords(&nonce))
+
+	var extractedNonce Musig2Nonce
+	_, err := extraData.ExtractRecords(&extractedNonce)
+	require.NoError(t, err)
+
+	require.Equal(t, nonce, extractedNonce)
+}
+
+// TestMusig2NonceTypeDecodeInvalidLength ensures that decoding a Musig2Nonce
+// TLV with an invalid length (anything other than 66 bytes) fails with an
+// error.
+func TestMusig2NonceTypeDecodeInvalidLength(t *testing.T) {
+	t.Parallel()
+
+	nonce := makeNonce()
+
+	var extraData ExtraOpaqueData
+	require.NoError(t, extraData.PackRecords(&nonce))
+
+	// Corrupt the TLV length field to simulate malformed input.
+	// Byte 1 contains the varint size encoding. Since 66 bytes fits into
+	// a single varint byte, we can directly modify extraData[1].
+	extraData[1] = musig2.PubNonceSize + 1
+
+	var out Musig2Nonce
+	_, err := extraData.ExtractRecords(&out)
+	require.Error(t, err)
+
+	extraData[1] = musig2.PubNonceSize - 1
+
+	_, err = extraData.ExtractRecords(&out)
+	require.Error(t, err)
+}