itest+lntest: add coordinator pattern test for combined nonce

Add integration test for MuSig2RegisterCombinedNonce and
MuSig2GetCombinedNonce RPCs to verify the coordinator pattern workflow.

The test:
- Creates three signing sessions without initial nonce exchange
- Manually aggregates nonces using the coordinator pattern (btcec musig2)
- Tests v0.4.0 returns unsupported errors (as expected)
- Tests v1.0.0rc2 successfully registers and retrieves combined nonces
- Verifies mutual exclusivity (error: already have all nonces)
- Completes a full signing flow to ensure signatures are valid

Also adds the required RPC harness wrapper methods to lntest/rpc/signer.go for
the new RPCs and adds MuSig2RegisterNoncesErr wrapper for error testing.

lntest/rpc/signer.go

@@ -130,6 +130,80 @@ func (h *HarnessRPC) MuSig2RegisterNonces(
 	return resp
 }
 
+// MuSig2RegisterNoncesErr makes a RPC call to the node's SignerClient and
+// asserts an error is returned.
+func (h *HarnessRPC) MuSig2RegisterNoncesErr(
+	req *signrpc.MuSig2RegisterNoncesRequest) error {
+
+	ctxt, cancel := context.WithTimeout(h.runCtx, DefaultTimeout)
+	defer cancel()
+
+	_, err := h.Signer.MuSig2RegisterNonces(ctxt, req)
+	require.Error(h, err, "expected error from MuSig2RegisterNonces")
+
+	return err
+}
+
+// MuSig2RegisterCombinedNonce makes a RPC call to the node's SignerClient and
+// asserts.
+//
+//nolint:ll
+func (h *HarnessRPC) MuSig2RegisterCombinedNonce(
+	req *signrpc.MuSig2RegisterCombinedNonceRequest) *signrpc.MuSig2RegisterCombinedNonceResponse {
+
+	ctxt, cancel := context.WithTimeout(h.runCtx, DefaultTimeout)
+	defer cancel()
+
+	resp, err := h.Signer.MuSig2RegisterCombinedNonce(ctxt, req)
+	h.NoError(err, "MuSig2RegisterCombinedNonce")
+
+	return resp
+}
+
+// MuSig2RegisterCombinedNonceErr makes a RPC call to the node's SignerClient
+// and asserts an error is returned.
+func (h *HarnessRPC) MuSig2RegisterCombinedNonceErr(
+	req *signrpc.MuSig2RegisterCombinedNonceRequest) error {
+
+	ctxt, cancel := context.WithTimeout(h.runCtx, DefaultTimeout)
+	defer cancel()
+
+	_, err := h.Signer.MuSig2RegisterCombinedNonce(ctxt, req)
+	require.Error(h, err, "expected error from MuSig2RegisterCombinedNonce")
+
+	return err
+}
+
+// MuSig2GetCombinedNonce makes a RPC call to the node's SignerClient and
+// asserts.
+//
+//nolint:ll
+func (h *HarnessRPC) MuSig2GetCombinedNonce(
+	req *signrpc.MuSig2GetCombinedNonceRequest) *signrpc.MuSig2GetCombinedNonceResponse {
+
+	ctxt, cancel := context.WithTimeout(h.runCtx, DefaultTimeout)
+	defer cancel()
+
+	resp, err := h.Signer.MuSig2GetCombinedNonce(ctxt, req)
+	h.NoError(err, "MuSig2GetCombinedNonce")
+
+	return resp
+}
+
+// MuSig2GetCombinedNonceErr makes a RPC call to the node's SignerClient and
+// asserts an error is returned.
+func (h *HarnessRPC) MuSig2GetCombinedNonceErr(
+	req *signrpc.MuSig2GetCombinedNonceRequest) error {
+
+	ctxt, cancel := context.WithTimeout(h.runCtx, DefaultTimeout)
+	defer cancel()
+
+	_, err := h.Signer.MuSig2GetCombinedNonce(ctxt, req)
+	require.Error(h, err, "expected error from MuSig2GetCombinedNonce")
+
+	return err
+}
+
 // MuSig2Sign makes a RPC call to the node's SignerClient and asserts.
 func (h *HarnessRPC) MuSig2Sign(
 	req *signrpc.MuSig2SignRequest) *signrpc.MuSig2SignResponse {