Map xid8 to pgtype.Uint64 for pgx/v5

Previously xid8 columns generated interface{} because only xid was
mapped. Fixes #4290.

Author: claude

internal/codegen/golang/postgresql_type.go

@@ -503,6 +503,11 @@ func postgresType(req *plugin.GenerateRequest, options *opts.Options, col *plugi
 			return "pgtype.XID"
 		}
 
+	case "xid8":
+		if driver == opts.SQLDriverPGXV5 {
+			return "pgtype.Uint64"
+		}
+
 	case "box":
 		if driver.IsPGX() {
 			return "pgtype.Box"

internal/endtoend/testdata/cid_oid_tid_xid/pgx/v5/go/models.go

@@ -4,9 +4,11 @@ CREATE TABLE test_table
     v_oid_null     oid,
     v_tid_null     tid,
     v_xid_null     xid,
+    v_xid8_null    xid8,
     v_cid          cid     not null,
     v_oid          oid     not null,
     v_tid          tid     not null,
-    v_xid          xid     not null
+    v_xid          xid     not null,
+    v_xid8         xid8    not null
 );