feat: Added support for enums as arguments for function tools (#3088)

* feat: Added support for enums as arguments for function tools

* feat: Add default value support for function tools
fix: Add more test cases inside `test_build_function_declaration.py` for passing Enums as arguments

* fix: format code with pyink

---------

Co-authored-by: Wei Sun (Jack) <weisun@google.com>
Co-authored-by: Yvonne Yu <150068659+yyyu-google@users.noreply.github.com>

tests/unittests/tools/test_build_function_declaration.py

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from enum import Enum
 from typing import Dict
 from typing import List
 
@@ -22,6 +23,7 @@ from google.genai import types
 # TODO: crewai requires python 3.10 as minimum
 # from crewai_tools import FileReadTool
 from pydantic import BaseModel
+import pytest
 
 
 def test_string_input():
@@ -220,6 +222,34 @@ def test_list():
   assert function_decl.parameters.properties['input_dir'].items.type == 'OBJECT'
 
 
+def test_enums():
+
+  class InputEnum(Enum):
+    AGENT = 'agent'
+    TOOL = 'tool'
+
+  def simple_function(input: InputEnum = InputEnum.AGENT):
+    return input.value
+
+  function_decl = _automatic_function_calling_util.build_function_declaration(
+      func=simple_function
+  )
+
+  assert function_decl.name == 'simple_function'
+  assert function_decl.parameters.type == 'OBJECT'
+  assert function_decl.parameters.properties['input'].type == 'STRING'
+  assert function_decl.parameters.properties['input'].default == 'agent'
+  assert function_decl.parameters.properties['input'].enum == ['agent', 'tool']
+
+  def simple_function_with_wrong_enum(input: InputEnum = 'WRONG_ENUM'):
+    return input.value
+
+  with pytest.raises(ValueError):
+    _automatic_function_calling_util.build_function_declaration(
+        func=simple_function_with_wrong_enum
+    )
+
+
 def test_basemodel_list():
   class ChildInput(BaseModel):
     input_str: str