fix: #3601471 Ignore unknown client-supplied asset library extensions in LibraryDiscoveryCollector

Catch UnknownExtensionException around LibraryDiscoveryParser::buildByExtension() in LibraryDiscoveryCollector::getLibraryDefinitions() and return an empty library set for unknown extensions. Client-supplied ajax_page_state[libraries] can reference extensions that no longer exist; previously this surfaced as an uncaught exception (fatal / 500) instead of treating the library as not loaded. Other parser failures for known extensions still throw.

Closes #3601471