fix(cli): omit senderIPAddress on master's /addGroup payload

The speaker's GroupService state machine uses the presence of
<senderIPAddress> in the addGroup payload to decide whether it should
form the group as master or join as slave: "SenderIp is provided, I am
the slave". Sending the same XML to both speakers (with senderIP set to
the master's IP) made the master also conclude it was the slave, enter
AddingSlave, time out after 5 s waiting for a master that never
confirmed, and revert. The slave briefly showed GROUP_OK before
following the master back to NoGroup -- the "stereo pair appears for a
few seconds, then disappears" symptom reported in #252.

Send two distinct payloads from propagateAddGroup: the master receives
the base request with no senderIPAddress, the slave receives a copy
with senderIPAddress set to the master's IP. The base request built by
createGroup no longer carries senderIPAddress; the per-role injection
is contained inside propagateAddGroup where the master/slave roles are
unambiguous.

Update TestPropagateAddGroup_BothSucceed to assert the master's body
has no <senderIPAddress> while the slave's body does, so any future
regression on either side fails the test.

Refs #252

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

cmd/soundtouch-cli/cmd_group.go

@@ -84,7 +84,8 @@ func createGroup(c *cli.Context) error {
 				{DeviceID: rightInfo.DeviceID, Role: "RIGHT", IPAddress: rightIP},
 			},
 		},
-		SenderIPAddress: leftIP,
+		// SenderIPAddress is intentionally omitted on the base request.
+		// propagateAddGroup adds it to the slave's copy only — see comment there.
 	}
 
 	leftClient, err := clientForHost(c, leftIP)
@@ -139,7 +140,21 @@ type addGroupOutcome struct {
 // propagateAddGroup POSTs /addGroup to both speakers concurrently and returns
 // the (LEFT, RIGHT) outcomes. A non-GROUP_OK Status in the response is
 // reported as an error so callers don't have to re-inspect the body.
+//
+// The two POSTs carry different payloads: the master (LEFT) receives the base
+// request with no senderIPAddress so its state machine forms the group as the
+// master, while the slave (RIGHT) receives a copy with senderIPAddress set to
+// the master's IP so its state machine joins as the slave. Sending the same
+// payload to both makes both speakers think they're the slave — they enter
+// AddingSlave, wait for a master that never confirms, time out after 5 s, and
+// revert (issue #252).
 func propagateAddGroup(left, right *client.Client, leftIP, rightIP string, req *models.Group) (addGroupOutcome, addGroupOutcome) {
+	masterReq := *req
+	masterReq.SenderIPAddress = ""
+
+	slaveReq := *req
+	slaveReq.SenderIPAddress = leftIP
+
 	var (
 		wg                sync.WaitGroup
 		leftOut, rightOut addGroupOutcome
@@ -150,13 +165,13 @@ func propagateAddGroup(left, right *client.Client, leftIP, rightIP string, req *
 	go func() {
 		defer wg.Done()
 
-		leftOut = postAddGroup(left, leftIP, req)
+		leftOut = postAddGroup(left, leftIP, &masterReq)
 	}()
 
 	go func() {
 		defer wg.Done()
 
-		rightOut = postAddGroup(right, rightIP, req)
+		rightOut = postAddGroup(right, rightIP, &slaveReq)
 	}()
 
 	wg.Wait()

cmd/soundtouch-cli/cmd_group_test.go

@@ -61,7 +61,8 @@ func sampleGroupRequest(leftIP, rightIP string) *models.Group {
 				{DeviceID: "F45EAB3115DA", Role: "RIGHT", IPAddress: rightIP},
 			},
 		},
-		SenderIPAddress: leftIP,
+		// senderIPAddress is intentionally not set here; propagateAddGroup
+		// adds it to the slave's copy only.
 	}
 }
 
@@ -95,19 +96,30 @@ func TestPropagateAddGroup_BothSucceed(t *testing.T) {
 		t.Errorf("RIGHT group = %+v, want status=GROUP_OK", rightOut.group)
 	}
 
-	// Both speakers must have received the same payload, including senderIPAddress.
-	for _, bodies := range []*[]string{leftBodies, rightBodies} {
+	// Both speakers must have received the roles, but only the slave's payload
+	// carries senderIPAddress — see propagateAddGroup for the why.
+	for label, bodies := range map[string]*[]string{"LEFT": leftBodies, "RIGHT": rightBodies} {
 		if len(*bodies) != 1 {
-			t.Fatalf("expected exactly one POST, got %d", len(*bodies))
+			t.Fatalf("%s: expected exactly one POST, got %d", label, len(*bodies))
 		}
 
 		body := (*bodies)[0]
-		for _, want := range []string{"<role>LEFT</role>", "<role>RIGHT</role>", "<senderIPAddress>192.168.1.131</senderIPAddress>"} {
+		for _, want := range []string{"<role>LEFT</role>", "<role>RIGHT</role>"} {
 			if !strings.Contains(body, want) {
-				t.Errorf("body missing %q\nbody:\n%s", want, body)
+				t.Errorf("%s body missing %q\nbody:\n%s", label, want, body)
 			}
 		}
 	}
+
+	leftBody := (*leftBodies)[0]
+	if strings.Contains(leftBody, "<senderIPAddress>") {
+		t.Errorf("LEFT (master) body must NOT carry <senderIPAddress>, otherwise the master flips into slave mode (issue #252)\nbody:\n%s", leftBody)
+	}
+
+	rightBody := (*rightBodies)[0]
+	if !strings.Contains(rightBody, "<senderIPAddress>192.168.1.131</senderIPAddress>") {
+		t.Errorf("RIGHT (slave) body must carry <senderIPAddress>192.168.1.131</senderIPAddress>\nbody:\n%s", rightBody)
+	}
 }
 
 func TestPropagateAddGroup_RightFails(t *testing.T) {