Address review comments

- data_availability_checker.rs: use !gloas_enabled() instead of < ForkName::Gloas
  (jimmygchen, dapplion).
- beacon_chain.rs: get_data_columns checks data_availability_checker first,
  then pending_payload_cache (dapplion).
- pending_components.rs: merge_data_columns drops the unused Result return
  (jimmygchen). num_completed_columns uses filter() instead of filter_map (jimmygchen).
- pending_column.rs: TODO marker on the hard-coded Gloas variant in
  try_to_sidecar (jimmygchen).
- pending_payload_cache/mod.rs: gloas_spec test helper collapsed to
  ForkName::Gloas.make_genesis_spec(E::default_spec()) (jimmygchen).
- gossip_methods.rs / sync/manager.rs: replace UnknownBlockHashFromAttestation
  fallback with TODO(gloas) for proper Gloas lookup sync (dapplion).

beacon_node/network/src/sync/manager.rs

@@ -906,6 +906,10 @@ impl<T: BeaconChainTypes> SyncManager<T> {
                         );
                     }
                     DataColumnSidecar::Gloas(_) => {
+                        // TODO(gloas): proper lookup sync for Gloas. Routing into
+                        // `handle_unknown_block_root` here mixes column processing with the
+                        // single-block-lookup path; the Gloas column-arrives-before-block
+                        // case wants its own queue/wakeup.
                         debug!(%block_root, "Received unknown block data column message");
                         self.handle_unknown_block_root(peer_id, block_root);
                     }