test: add formatter tests

- Add comprehensive formatter test suite with 38 tests
- Test multiline assignments, FB calls, control structures
- Test IF, FOR, WHILE, CASE, REPEAT statements
- Test regions, structs, var sections
- Update AGENTS.md with formatter test command

test/run_tests.lua

@@ -28,6 +28,7 @@ local function run_all_tests()
   run_test_file("test_udt.lua")
   run_test_file("test_db.lua")
   run_test_file("test_fb.lua")
+  run_test_file("test_formatter.lua")
   run_test_file("test_plc_json.lua")
   run_test_file("test_integration.lua")