fix: prevent catalog scan use-after-free (1.5)

[staticlibs]

This is a backport of the PR #501 to v1.5-variegata stable branch.

Resolves #502

PostgresCatalogSet::Scan hands its callback a bare CatalogEntry& without pinning it into the calling transaction first. GetEntry and CreateEntry both keep the underlying shared_ptr<CatalogEntry> alive in PostgresTransaction::referenced_entries for the rest of the transaction. Scan does not.

If pg_clear_cache runs after a Scan but before every reference handed out by that Scan is done being used, ClearEntries drops the shared_ptr for those entries and destroys them. Anything still holding a reference from the earlier Scan is then reading freed memory; i.e., funcs from Duckdb core like duckdb_tables() or duckdb_vies()

Pinning every entry into the calling transaction inside Scan's existing lock scope, the same way GetEntry and CreateEntry already do:

for (auto &entry : entries) {
        transaction.ReferenceEntry(entry.second);
        callback(*entry.second);
}

ReferenceEntry/referenced_entries is already the established solution for this exact problem. Scan just wasn't using it yet