Build: Fix Gutenberg copy script missing constants.php and incorrect base URL.

The copy script was not including `constants.php` in the list of PHP infrastructure
files to sync from Gutenberg's build output to Core. Additionally, the base URL
passed to the Gutenberg build had the trailing slash outside the `includes_url()`
call, resulting in URLs like `build/pages` being generated as `buildpages`.

Props youknowriad.
Fixes #64656.

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

Author: youknowriad

tools/gutenberg/build-gutenberg.js

@@ -145,8 +145,8 @@ async function main() {
 		// On Unix, arguments are passed directly without shell parsing
 		const baseUrlArg =
 			process.platform === 'win32'
-				? '--base-url="includes_url( \'build\' )"'
-				: "--base-url=includes_url( 'build' )";
+				? '--base-url="includes_url( \'build/\' )"'
+				: "--base-url=includes_url( 'build/' )";
 
 		await exec( 'npm', [ 'run', 'build', '--', '--fast', baseUrlArg ], {
 			cwd: gutenbergDir,

tools/gutenberg/copy-gutenberg-build.js

@@ -39,7 +39,7 @@ const COPY_CONFIG = {
 	// PHP infrastructure files (to wp-includes/build/)
 	phpInfrastructure: {
 		destination: 'build',
-		files: [ 'routes.php', 'pages.php' ],
+		files: [ 'routes.php', 'pages.php', 'constants.php' ],
 		directories: [ 'pages', 'routes' ],
 	},