chore: add diagnostic logging for debug mode verification

This temporary logging will help verify that:
- EnvironmentType is correctly detected as STAGING
- isDebugEnabled() returns false for STAGING
- ResponseErrorRenderer receives isDebugMode=false

Remove after verification is complete.

src/Framework/ExceptionHandling/Renderers/ResponseErrorRenderer.php

@@ -278,6 +278,9 @@ final readonly class ResponseErrorRenderer implements ErrorRenderer
         );
 
         $debugInfo = '';
+        // SECURITY FIX: Only show debug info in development mode
+        // Log to error_log for diagnostic purposes (can be removed after verification)
+        error_log("[ResponseErrorRenderer] isDebugMode={$this->isDebugMode}, APP_ENV=" . ($_ENV['APP_ENV'] ?? 'unknown'));
         if ($this->isDebugMode) {
             $debugInfo = $this->generateDebugSection($exception, $contextProvider);
         }