Add test for Router.get_valid_args, fix router code coverage encoding (#19797)

- Add test_get_valid_args in test_router_helper_utils.py to cover get_valid_args
- Use encoding='utf-8' in router_code_coverage.py for cross-platform file reads

tests/code_coverage_tests/router_code_coverage.py

@@ -6,7 +6,7 @@ def get_function_names_from_file(file_path):
     """
     Extracts all function names from a given Python file.
     """
-    with open(file_path, "r") as file:
+    with open(file_path, "r", encoding="utf-8") as file:
         tree = ast.parse(file.read())
 
     function_names = []
@@ -45,7 +45,7 @@ def get_all_functions_called_in_tests(base_dir):
                 if file.endswith(".py") and "router" in file.lower():
                     print("file: ", file)
                     file_path = os.path.join(root, file)
-                    with open(file_path, "r") as f:
+                    with open(file_path, "r", encoding="utf-8") as f:
                         try:
                             tree = ast.parse(f.read())
                         except SyntaxError: