Display unexpected plugin output during activation with --debug

features/plugin-activate.feature

@@ -154,3 +154,26 @@ Feature: Activate WordPress plugins
       Success:
       """
     And the return code should be 0
+
+  Scenario: Activating a plugin that generates unexpected output shows the output in debug mode
+    Given a wp-content/plugins/output-plugin.php file:
+      """
+      <?php
+      /**
+       * Plugin Name: Output Plugin
+       * Description: This plugin generates unexpected output during activation
+       * Author: WP-CLI tests
+       */
+      echo "Unexpected output from plugin activation";
+      """
+
+    When I try `wp plugin activate output-plugin --debug`
+    Then STDERR should contain:
+      """
+      Warning: Failed to activate plugin. The plugin generated unexpected output.
+      """
+    And STDERR should contain:
+      """
+      Debug (plugin): Unexpected output: Unexpected output from plugin activation
+      """
+    And the return code should be 1

src/Plugin_Command.php

@@ -401,6 +401,16 @@ public function activate( $args, $assoc_args = [] ) {
 				$message = wp_strip_all_tags( $message );
 				$message = str_replace( 'Error: ', '', $message );
 				WP_CLI::warning( "Failed to activate plugin. {$message}" );
+				// If the error is due to unexpected output, display it for debugging
+				if ( 'unexpected_output' === $result->get_error_code() ) {
+					/**
+					 * @var string $output
+					 */
+					$output = $result->get_error_data();
+					if ( ! empty( $output ) ) {
+						WP_CLI::debug( "Unexpected output: {$output}", 'plugin' );
+					}
+				}
 				++$errors;
 			} else {
 				$this->active_output( $plugin->name, $plugin->file, $network_wide, 'activate' );