fix: guard WP_Error cost access + add missing test constants

- Wrap cost_usd accumulation in is_wp_error() guard so WP_Error
  returns from generate_image_a/b don't crash on array access
- Add PRAUTOBLOGGER_DEFAULT_IMAGE_STYLE_SUFFIX to test bootstrap
- Add is_wp_error, sanitize_title, esc_html__ mocks to ImagePipelineTest

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/unit/Core/ImagePipelineTest.php

@@ -19,6 +19,9 @@ class ImagePipelineTest extends BaseTestCase {
 		parent::setUp();
 
 		// Mock WordPress functions used by the pipeline and its dependencies.
+		Functions\when( 'is_wp_error' )->alias( function ( $thing ) {
+			return $thing instanceof \WP_Error;
+		} );
 		Functions\when( 'wp_generate_uuid4' )->justReturn( 'test-uuid' );
 		Functions\when( 'get_option' )->alias( function ( $key, $default = false ) {
 			static $options = [
@@ -32,6 +35,8 @@ class ImagePipelineTest extends BaseTestCase {
 			return trim( strip_tags( $str ) );
 		} );
 		Functions\when( 'update_post_meta' )->justReturn( true );
+		Functions\when( 'sanitize_title' )->returnArg();
+		Functions\when( 'esc_html__' )->returnArg();
 	}
 
 	/**