fix: also guard X25519 ECDH against a nil/empty receiver keypair

Extend the ECDH nil-guard to the receiver: a nil *KeyPair or a zero-value
keypair (nil underlying private key) now returns an error instead of
panicking, matching the guard already applied to the peer key.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W7WcBfhKfPFyj2Gp4H6s7R

Author: claude

multikey/x25519/x25519.go

@@ -113,9 +113,12 @@ func (k *KeyPair) KeyDID() did.DID { return k.Public().KeyDID() }
 
 // ECDH performs X25519 Diffie-Hellman between this private key and the peer's
 // public key, returning the raw shared secret. It returns an error rather than
-// panicking when peer is nil or carries no public key, since peer keys may come
-// from external input.
+// panicking when either the keypair or the peer is nil or carries no key, since
+// peer keys may come from external input.
 func (k *KeyPair) ECDH(peer *PublicKey) ([]byte, error) {
+	if k == nil || k.priv == nil {
+		return nil, fmt.Errorf("nil private key")
+	}
 	if peer == nil || peer.pub == nil {
 		return nil, fmt.Errorf("nil peer public key")
 	}

multikey/x25519/x25519_test.go

@@ -156,4 +156,13 @@ func TestECDH(t *testing.T) {
 		_, err = alice.ECDH(&x25519.PublicKey{})
 		require.Error(t, err)
 	})
+
+	t.Run("returns an error for a nil or empty keypair instead of panicking", func(t *testing.T) {
+		var nilKeyPair *x25519.KeyPair
+		_, err := nilKeyPair.ECDH(alice.Public())
+		require.Error(t, err)
+		// A zero-value keypair (nil underlying key) is also rejected.
+		_, err = (&x25519.KeyPair{}).ECDH(alice.Public())
+		require.Error(t, err)
+	})
 }