graph/db: add gossip version parameter to ForEachChannel

Update ForEachChannel to accept a gossip version parameter, allowing
callers to specify which gossip version's channels should be iterated.
This change prepares the graph database for supporting multiple gossip
versions while maintaining backward compatibility.

The KVStore implementation validates that only GossipVersion1 is
requested, returning ErrVersionNotSupportedForKVDB for other versions.
The SQLStore implementation validates known versions and passes the
version through to the underlying paginated query.

Update VersionedGraph to include a ForEachChannel method that
automatically uses its configured gossip version, and update the
DescribeGraph RPC handler to use the v1Graph instead of the global
graphDB.

graph/db/sql_store.go

@@ -1610,11 +1610,16 @@ func (s *SQLStore) ForEachChannelCacheable(cb func(*models.CachedEdgeInfo,
 //
 // NOTE: part of the Store interface.
 func (s *SQLStore) ForEachChannel(ctx context.Context,
-	cb func(*models.ChannelEdgeInfo, *models.ChannelEdgePolicy,
-		*models.ChannelEdgePolicy) error, reset func()) error {
+	v lnwire.GossipVersion, cb func(*models.ChannelEdgeInfo,
+		*models.ChannelEdgePolicy, *models.ChannelEdgePolicy) error,
+	reset func()) error {
+
+	if !isKnownGossipVersion(v) {
+		return fmt.Errorf("unsupported gossip version: %d", v)
+	}
 
 	return s.db.ExecTx(ctx, sqldb.ReadTxOpt(), func(db SQLQueries) error {
-		return forEachChannelWithPolicies(ctx, db, s.cfg, cb)
+		return forEachChannelWithPolicies(ctx, db, s.cfg, v, cb)
 	}, reset)
 }
 
@@ -5973,8 +5978,8 @@ func forEachNodePaginated(ctx context.Context, cfg *sqldb.QueryConfig,
 // forEachChannelWithPolicies executes a paginated query to process each channel
 // with policies in the graph.
 func forEachChannelWithPolicies(ctx context.Context, db SQLQueries,
-	cfg *SQLStoreConfig, processChannel func(*models.ChannelEdgeInfo,
-		*models.ChannelEdgePolicy,
+	cfg *SQLStoreConfig, v lnwire.GossipVersion,
+	processChannel func(*models.ChannelEdgeInfo, *models.ChannelEdgePolicy,
 		*models.ChannelEdgePolicy) error) error {
 
 	type channelBatchIDs struct {
@@ -5988,7 +5993,7 @@ func forEachChannelWithPolicies(ctx context.Context, db SQLQueries,
 
 		return db.ListChannelsWithPoliciesPaginated(
 			ctx, sqlc.ListChannelsWithPoliciesPaginatedParams{
-				Version: int16(lnwire.GossipVersion1),
+				Version: int16(v),
 				ID:      lastID,
 				Limit:   limit,
 			},