Switch Linea Besu from SNAP to FULL sync mode

SNAP sync is broken for Linea - it picks an old pivot block (~24.7M) that
no peers can serve world state for. This causes:
- World state download stuck with 0 pending requests
- Maru unable to push blocks to EL without complete world state
- Node stuck returning block 0 for "latest"

FULL sync executes every block from genesis. It's slower but reliable
and allows maru to drive the sync via engine API.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

linea/besu/linea-mainnet-besu-pruned-bonsai.yml

@@ -78,7 +78,7 @@ services:
       - --rpc-ws-enabled=true
       - --rpc-ws-host=0.0.0.0
       - --rpc-ws-port=8546
-      - --sync-mode=SNAP
+      - --sync-mode=FULL
     restart: unless-stopped
     stop_grace_period: 5m
     networks: