chore: Add coverage for pipe union list annotations

Merge #6160

Add regression coverage for optional list parameters written with Python pipe union syntax, covering the List[str] | None = None signature shape. Refs #3591.

PiperOrigin-RevId: 941157723

Author: vivekjm

tests/unittests/tools/test_function_tool_with_import_annotations.py

@@ -159,6 +159,26 @@ def test_function(str_param: str, int_param: int, any_param: Any) -> str:
   assert declaration.response.type == types.Type.STRING
 
 
+def test_pipe_union_list_annotation_parameter_vertex():
+  """Test function with pipe union list parameter annotation."""
+
+  def test_function(file_patterns: list[str] | None = None) -> None:
+    """A test function that accepts optional file patterns."""
+    pass
+
+  declaration = _automatic_function_calling_util.from_function_with_options(
+      test_function, GoogleLLMVariant.VERTEX_AI
+  )
+
+  assert declaration.name == 'test_function'
+  assert declaration.parameters.type == 'OBJECT'
+  file_patterns_schema = declaration.parameters.properties['file_patterns']
+  assert file_patterns_schema.type == types.Type.ARRAY
+  assert file_patterns_schema.items.type == types.Type.STRING
+  assert file_patterns_schema.nullable
+  assert declaration.parameters.required == []
+
+
 def test_string_annotation_no_params_vertex():
   """Test function with no parameters but string annotation return."""