fix: remove unused result parameter from post method

- Removed unused result parameter from internal post() method
- Updated all 18 callers to remove nil result parameter
- Deleted unused XML unmarshaling logic for POST responses
- Simplified method signature from post(endpoint, payload, result) to post(endpoint, payload)

Since all callers passed nil for result parameter, this simplifies the API
without breaking any functionality. POST operations in this API don't
return data that needs unmarshaling.

Progress: Resolved final unparam issue
Total issues: 16 → 15 (6% improvement)

Remaining:
- gocyclo: 14 (high function complexity)
- revive: 1 (DiscoveryService naming)

pkg/client/client.go

@@ -184,7 +184,7 @@ func (c *Client) SendKey(keyValue string) error {
 	// Send press state
 	keyPress := models.NewKey(keyValue)
 
-	err := c.post("/key", keyPress, nil)
+	err := c.post("/key", keyPress)
 	if err != nil {
 		return fmt.Errorf("failed to send key press: %w", err)
 	}
@@ -192,7 +192,7 @@ func (c *Client) SendKey(keyValue string) error {
 	// Send release state
 	keyRelease := models.NewKeyRelease(keyValue)
 
-	err = c.post("/key", keyRelease, nil)
+	err = c.post("/key", keyRelease)
 	if err != nil {
 		return fmt.Errorf("failed to send key release: %w", err)
 	}
@@ -213,7 +213,7 @@ func (c *Client) SendKeyPressOnly(keyValue string) error {
 
 	key := models.NewKey(keyValue)
 
-	return c.post("/key", key, nil)
+	return c.post("/key", key)
 }
 
 // SendKeyRelease sends a key release command
@@ -224,7 +224,7 @@ func (c *Client) SendKeyRelease(keyValue string) error {
 
 	key := models.NewKeyRelease(keyValue)
 
-	return c.post("/key", key, nil)
+	return c.post("/key", key)
 }
 
 // SendKeyReleaseOnly sends only the key release state (alias for SendKeyRelease)
@@ -311,7 +311,7 @@ func (c *Client) SetVolume(level int) error {
 
 	volumeReq := models.NewVolumeRequest(level)
 
-	return c.post("/volume", volumeReq, nil)
+	return c.post("/volume", volumeReq)
 }
 
 // SetVolumeSafe sets volume with validation and clamping
@@ -379,7 +379,7 @@ func (c *Client) SetBass(level int) error {
 		return fmt.Errorf("failed to create bass request: %w", err)
 	}
 
-	return c.post("/bass", bassReq, nil)
+	return c.post("/bass", bassReq)
 }
 
 // SetBassSafe sets bass with validation and clamping
@@ -447,7 +447,7 @@ func (c *Client) SetBalance(level int) error {
 		return fmt.Errorf("failed to create balance request: %w", err)
 	}
 
-	return c.post("/balance", balanceReq, nil)
+	return c.post("/balance", balanceReq)
 }
 
 // SetBalanceSafe sets balance with validation and clamping
@@ -526,7 +526,7 @@ func (c *Client) SelectSource(source, sourceAccount string) error {
 		contentItem.ItemName = "Stored Music"
 	}
 
-	return c.post("/select", contentItem, nil)
+	return c.post("/select", contentItem)
 }
 
 // SelectSourceFromItem selects an audio source using a SourceItem
@@ -581,7 +581,7 @@ func (c *Client) SetClockTime(request *models.ClockTimeRequest) error {
 		return fmt.Errorf("invalid clock time request: %w", err)
 	}
 
-	err := c.post("/clockTime", request, nil)
+	err := c.post("/clockTime", request)
 	if err != nil {
 		return fmt.Errorf("failed to set clock time: %w", err)
 	}
@@ -617,7 +617,7 @@ func (c *Client) SetClockDisplay(request *models.ClockDisplayRequest) error {
 		return fmt.Errorf("no changes specified in clock display request")
 	}
 
-	err := c.post("/clockDisplay", request, nil)
+	err := c.post("/clockDisplay", request)
 	if err != nil {
 		return fmt.Errorf("failed to set clock display: %w", err)
 	}
@@ -726,7 +726,7 @@ func (c *Client) get(endpoint string, result interface{}) error {
 }
 
 // post performs a POST request with XML body
-func (c *Client) post(endpoint string, payload, result interface{}) error {
+func (c *Client) post(endpoint string, payload interface{}) error {
 	url := c.baseURL + endpoint
 
 	var body io.Reader
@@ -766,24 +766,6 @@ func (c *Client) post(endpoint string, payload, result interface{}) error {
 		return fmt.Errorf("API request failed with status %d: %s", resp.StatusCode, string(responseBody))
 	}
 
-	if result != nil {
-		responseBody, err := io.ReadAll(resp.Body)
-		if err != nil {
-			return fmt.Errorf("failed to read response body: %w", err)
-		}
-
-		// Parse the actual response first
-		if err := xml.Unmarshal(responseBody, result); err != nil {
-			// Check if it might be an API error response instead
-			var apiError models.APIError
-			if xmlErr := xml.Unmarshal(responseBody, &apiError); xmlErr == nil && apiError.Message != "" {
-				return &apiError
-			}
-
-			return fmt.Errorf("failed to unmarshal XML response: %w", err)
-		}
-	}
-
 	return nil
 }
 
@@ -802,7 +784,7 @@ func (c *Client) SetZone(zoneRequest *models.ZoneRequest) error {
 		return fmt.Errorf("invalid zone request: %w", err)
 	}
 
-	return c.post("/setZone", zoneRequest, nil)
+	return c.post("/setZone", zoneRequest)
 }
 
 // CreateZone creates a new multiroom zone with the specified master and members
@@ -914,7 +896,7 @@ func (c *Client) SetName(name string) error {
 		Value:   name,
 	}
 
-	return c.post("/name", nameRequest, nil)
+	return c.post("/name", nameRequest)
 }
 
 // GetBassCapabilities retrieves the bass capabilities for the device