BigPipe accumulates duplicate library entries across placeholder renders

Problem/Motivation

When BigPipe renders multiple placeholders, it accumulates already-loaded libraries using array_merge() without deduplication:

$cumulative_assets->setAlreadyLoadedLibraries(array_merge(
    $cumulative_assets->getAlreadyLoadedLibraries(),
    $html_response->getAttachments()['library']
));

If the same library is attached in both the main page render and a lazy builder response, it accumulates twice. On a subsequent placeholder render, LibraryDependencyResolver::getMinimalRepresentativeSubset() asserts uniqueness on the resulting list and throws an AssertionError, which escapes BigPipe's catch (\Exception) blocks uncaught and leaves the response in an undefined state.

Observed duplicate libraries on sites running Gin admin theme with Flag module: gin/core_navigation, core/components.navigation--toolbar-button, flag/flag.link, flag/flag.link_ajax.

Steps to reproduce

1. Enable big_pipe on a site with the Gin admin theme and the Flag module.
2. Enable PHP assertions (zend.assertions = 1, assert.exception = 1).
3. Visit any page as an admin user that renders Flag link lazy builders alongside the Gin navigation toolbar.
4. Observe AssertionError: $libraries can't contain duplicate items from LibraryDependencyResolver::getMinimalRepresentativeSubset().

Proposed resolution

1. Deduplicate the cumulative library list:

$cumulative_assets->setAlreadyLoadedLibraries(array_values(array_unique(array_merge(
    $cumulative_assets->getAlreadyLoadedLibraries(),
    $html_response->getAttachments()['library']
))));

2. Replace catch (\Exception $e) with catch (\Throwable $e) in the placeholder rendering catch blocks in sendNoJsPlaceholders() and sendPlaceholders(), so that \Error subclasses are caught and logged via trigger_error() rather than escaping uncaught.