refactor: introduce constants for improved maintainability in tests and server configurations

- Added constants for test endpoints, usernames, and XML headers in client_test.go and device_certificates_test.go to enhance readability and reduce hardcoded values.
- Updated various test cases to utilize these constants, ensuring consistency across tests.
- Refactored imaging settings and server configurations to use defined constants for default values, improving clarity and maintainability in server/device.go and server/imaging.go.
- Enhanced comments throughout the code to clarify functionality and adhere to best practices.

imaging.go

@@ -142,7 +142,11 @@ func (c *Client) GetImagingSettings(ctx context.Context, videoSourceToken string
 }
 
 // SetImagingSettings sets imaging settings for a video source.
-func (c *Client) SetImagingSettings(ctx context.Context, videoSourceToken string, settings *ImagingSettings, forcePersistence bool) error {
+//
+//nolint:funlen // SetImagingSettings has many statements due to building complex imaging settings request
+func (c *Client) SetImagingSettings(
+	ctx context.Context, videoSourceToken string, settings *ImagingSettings, forcePersistence bool,
+) error {
 	endpoint := c.imagingEndpoint
 	if endpoint == "" {
 		endpoint = c.endpoint