fix: remove premature on_valid_payload_envelope_received + disable backfill for GLOaS

1. Remove on_valid_payload_envelope_received call before process_block
   in chain segment import. The block isn't in fork choice yet, so it
   always fails with NodeUnknown. import_envelope_from_range_sync
   handles this correctly after process_block.

2. Disable backfill sync when GLOaS is scheduled. Backfill calls
   into_available_block which panics on GLOaS RangeSyncBlock variant.
   Backfill for GLOaS is not yet implemented.

beacon_node/network/src/sync/backfill_sync/mod.rs

@@ -215,6 +215,11 @@ impl<T: BeaconChainTypes> BackFillSync<T> {
         &mut self,
         network: &mut SyncNetworkContext<T>,
     ) -> Result<SyncStart, BackFillError> {
+        // Skip backfill sync for GLOaS — not yet implemented for this fork.
+        if self.beacon_chain.spec.gloas_fork_epoch.is_some_and(|e| e != Epoch::max_value()) {
+            return Ok(SyncStart::NotSyncing);
+        }
+
         match self.state() {
             BackFillState::Syncing => {} // already syncing ignore.
             BackFillState::Paused => {