CVE-2026-52734: zebrad has unbounded memory leak in mempool download pipeline via timeout path cancel_handles retention

The mempool download pipeline’s cancel_handles map retains entries for transactions whose verification times out at the outer RATE_LIMIT_DELAY (73-second) boundary. The tokio::time::error::Elapsed error carries no payload, so the transaction ID is unrecoverable and the corresponding cancel_handles entry (including the full Gossip::Tx(UnminedTx), up to ~2 MB) is never removed. Entries accumulate monotonically with no upper bound or garbage collection, leading to eventual out-of-memory process termination.