explat: Fix Phan issues (#39443)

Add some missing `use` directives and add `WP_Error` to one return
declaration.

projects/packages/explat/.phan/baseline.php

@@ -8,15 +8,9 @@
  * (can be combined with --load-baseline)
  */
 return [
-    // # Issue statistics:
-    // PhanTypeArraySuspicious : 3 occurrences
-    // PhanTypeMismatchReturn : 3 occurrences
-    // PhanUndeclaredTypeParameter : 1 occurrence
-    // PhanUndeclaredTypeReturnType : 1 occurrence
-
+    // This baseline has no suppressions
     // Currently, file_suppressions and directory_suppressions are the only supported suppressions
     'file_suppressions' => [
-        'src/class-rest-controller.php' => ['PhanTypeArraySuspicious', 'PhanTypeMismatchReturn', 'PhanUndeclaredTypeParameter', 'PhanUndeclaredTypeReturnType'],
     ],
     // 'directory_suppressions' => ['src/directory_name' => ['PhanIssueName1', 'PhanIssueName2']] can be manually added if needed.
     // (directory_suppressions will currently be ignored by subsequent calls to --save-baseline, but may be preserved in future Phan releases)

projects/packages/explat/changelog/fix-phan-errors-in-packages-explat

@@ -0,0 +1,5 @@
+Significance: patch
+Type: fixed
+Comment: Add missing `use` and one return type to fix Phan issues. No change to functionality.
+
+

projects/packages/explat/src/class-rest-controller.php

@@ -11,6 +11,8 @@
 use Automattic\Jetpack\Connection\Client;
 use Automattic\Jetpack\Connection\Manager as Jetpack_Connection;
 use WP_Error;
+use WP_REST_Request;
+use WP_REST_Response;
 use WP_REST_Server;
 
 /**
@@ -71,7 +73,7 @@ public function register_rest_routes() {
 	 * Get the assignments for a given experiment and anon_id
 	 *
 	 * @param WP_REST_Request $request The REST request object.
-	 * @return WP_REST_Response
+	 * @return WP_REST_Response|WP_Error
 	 */
 	public function get_assignments( $request ) {
 		$response          = null;