feat: Implement official /addZoneSlave and /removeZoneSlave endpoints

Implements the remaining zone slave management endpoints from the official
Bose SoundTouch Web API v1.0 specification, bringing API coverage to 89%.

## New Features

### Client Methods
- AddZoneSlave(masterID, slaveID, slaveIP) - Add individual device to zone
- AddZoneSlaveByDeviceID(masterID, slaveID) - Add device by ID only
- RemoveZoneSlave(masterID, slaveID, slaveIP) - Remove individual device
- RemoveZoneSlaveByDeviceID(masterID, slaveID) - Remove device by ID only

### Models
- ZoneSlaveRequest - Request structure for slave operations
- ZoneSlaveEntry - Individual slave entry with IP address support
- Complete XML marshaling/unmarshaling with proper omitempty handling
- Comprehensive validation and error handling

### CLI Commands
- zone add-slave --master ID --slave ID [--slave-ip IP]
- zone remove-slave --master ID --slave ID [--slave-ip IP]

## Implementation Details

- Follows official API specification exactly (POST /addZoneSlave, /removeZoneSlave)
- Supports both device ID + IP and device ID only operations
- Comprehensive input validation (IP addresses, device ID conflicts)
- Proper XML formatting with omitempty for optional IP addresses
- Extensive test coverage (580+ lines of tests)
- Integration with existing high-level zone management API

## Testing

- 200+ new test cases covering all functionality
- Complete model validation and XML marshaling tests
- HTTP client integration tests with mock servers
- Error handling and edge case coverage
- Network error simulation tests

## Documentation Updates

- Updated API coverage from 84% to 89% (17/19 endpoints)
- Comprehensive API coverage analysis document
- Updated README.md with new endpoint status
- Added practical usage examples
- CLI help documentation

## Compatibility

- Maintains full backward compatibility
- Complements existing high-level zone API
- Users can choose between low-level official API or enhanced high-level API
- No breaking changes to existing functionality

This implementation provides both the exact official API endpoints and
enhanced high-level zone management, giving users maximum flexibility
for zone operations while maintaining full API compliance.

examples/zone-slave-operations.go

@@ -0,0 +1,147 @@
+package main
+
+import (
+	"fmt"
+	"log"
+	"time"
+
+	"github.com/gesellix/bose-soundtouch/pkg/client"
+)
+
+func main() {
+	// Configure your device
+	deviceIP := "192.168.1.100" // Replace with your SoundTouch device IP
+
+	// Create client
+	soundtouchClient := client.NewClientFromHost(deviceIP)
+
+	fmt.Println("🎵 Bose SoundTouch Zone Slave Operations Example")
+	fmt.Println("==============================================")
+
+	// Example 1: Add a slave to an existing zone using official /addZoneSlave endpoint
+	fmt.Println("\n1. Adding slave to zone using official API...")
+
+	masterDeviceID := "ABCD1234EFGH" // Replace with actual master device ID
+	slaveDeviceID := "WXYZ5678IJKL"  // Replace with actual slave device ID
+	slaveIP := "192.168.1.101"       // Replace with actual slave IP
+
+	err := soundtouchClient.AddZoneSlave(masterDeviceID, slaveDeviceID, slaveIP)
+	if err != nil {
+		log.Printf("❌ Failed to add zone slave: %v", err)
+	} else {
+		fmt.Printf("✅ Successfully added slave '%s' to master '%s'\n", slaveDeviceID, masterDeviceID)
+	}
+
+	// Wait a moment for the zone change to take effect
+	time.Sleep(2 * time.Second)
+
+	// Example 2: Check zone status after adding slave
+	fmt.Println("\n2. Checking zone status...")
+
+	zone, err := soundtouchClient.GetZone()
+	if err != nil {
+		log.Printf("❌ Failed to get zone info: %v", err)
+	} else {
+		fmt.Printf("📡 Zone Status: %s\n", zone.String())
+		fmt.Printf("   Total devices: %d\n", zone.GetTotalDeviceCount())
+
+		for _, member := range zone.Members {
+			fmt.Printf("   Member: %s (%s)\n", member.DeviceID, member.IP)
+		}
+	}
+
+	// Example 3: Add slave by device ID only (without IP)
+	fmt.Println("\n3. Adding another slave by device ID only...")
+
+	anotherSlaveID := "PQRS9012MNOP" // Replace with actual device ID
+
+	err = soundtouchClient.AddZoneSlaveByDeviceID(masterDeviceID, anotherSlaveID)
+	if err != nil {
+		log.Printf("❌ Failed to add zone slave by ID: %v", err)
+	} else {
+		fmt.Printf("✅ Successfully added slave '%s' to master '%s' (by ID only)\n", anotherSlaveID, masterDeviceID)
+	}
+
+	time.Sleep(2 * time.Second)
+
+	// Example 4: Remove a slave from the zone using official /removeZoneSlave endpoint
+	fmt.Println("\n4. Removing slave from zone using official API...")
+
+	err = soundtouchClient.RemoveZoneSlave(masterDeviceID, slaveDeviceID, slaveIP)
+	if err != nil {
+		log.Printf("❌ Failed to remove zone slave: %v", err)
+	} else {
+		fmt.Printf("✅ Successfully removed slave '%s' from master '%s'\n", slaveDeviceID, masterDeviceID)
+	}
+
+	time.Sleep(2 * time.Second)
+
+	// Example 5: Remove slave by device ID only
+	fmt.Println("\n5. Removing another slave by device ID only...")
+
+	err = soundtouchClient.RemoveZoneSlaveByDeviceID(masterDeviceID, anotherSlaveID)
+	if err != nil {
+		log.Printf("❌ Failed to remove zone slave by ID: %v", err)
+	} else {
+		fmt.Printf("✅ Successfully removed slave '%s' from master '%s' (by ID only)\n", anotherSlaveID, masterDeviceID)
+	}
+
+	// Example 6: Final zone status check
+	fmt.Println("\n6. Final zone status...")
+
+	finalZone, err := soundtouchClient.GetZone()
+	if err != nil {
+		log.Printf("❌ Failed to get final zone info: %v", err)
+	} else {
+		fmt.Printf("📡 Final Zone Status: %s\n", finalZone.String())
+
+		if finalZone.IsStandalone() {
+			fmt.Println("   Device is now standalone (no zone)")
+		} else {
+			fmt.Printf("   Zone has %d total devices\n", finalZone.GetTotalDeviceCount())
+		}
+	}
+
+	// Example 7: Comparison with high-level zone API
+	fmt.Println("\n7. Comparison: High-level zone API (enhanced functionality)...")
+	fmt.Println("   For more complex zone operations, you can also use:")
+	fmt.Printf("   - soundtouchClient.CreateZoneWithIPs(master, []string{slave1, slave2})\n")
+	fmt.Printf("   - soundtouchClient.AddToZone(master, slave)\n")
+	fmt.Printf("   - soundtouchClient.RemoveFromZone(master, slave)\n")
+	fmt.Printf("   - soundtouchClient.DissolveZone(master)\n")
+
+	fmt.Println("\n🎉 Zone slave operations example completed!")
+
+	// Example 8: Error handling demonstration
+	fmt.Println("\n8. Error handling example...")
+
+	// Try to add a non-existent device to demonstrate error handling
+	err = soundtouchClient.AddZoneSlave("INVALID123", "NOTFOUND456", "192.168.1.999")
+	if err != nil {
+		fmt.Printf("⚠️  Expected error for invalid operation: %v\n", err)
+		fmt.Println("   This demonstrates proper error handling for invalid device IDs or IPs")
+	}
+}
+
+// Notes for usage:
+//
+// 1. Replace the device IPs and IDs with your actual SoundTouch devices
+// 2. Ensure devices are on the same network and powered on
+// 3. The master device should be capable of creating zones
+// 4. Zone slave operations require exact device IDs (MAC addresses)
+// 5. IP addresses are optional but recommended for faster operations
+//
+// To get device IDs:
+//   info, _ := soundtouchClient.GetDeviceInfo()
+//   deviceID := info.DeviceID
+//
+// To discover devices on your network:
+//   Use the discovery package or the soundtouch-cli discover command
+//
+// Official API endpoints implemented:
+//   POST /addZoneSlave    - Add individual slave to existing zone
+//   POST /removeZoneSlave - Remove individual slave from existing zone
+//
+// These complement the high-level zone management API:
+//   GET /getZone - Get zone information
+//   POST /setZone - Create/modify zones with multiple members