Guard StaticReflectionParser against unreadable or non-file paths

StaticReflectionParser::parse() may be invoked with paths that are not readable files (such as directories encountered during theme or module hook discovery), causing file_get_contents() to emit warnings like “Permission denied”. This change adds a defensive guard to ensure parsing only proceeds when the resolved path is an existing, readable file, preventing warnings while preserving behavior for valid PHP class files and improving robustness across platforms.