fix(types): support IntEnum in callable tools

[MukundaKatta]

Summary

• handle Enum annotations directly in automatic callable schema parsing instead of falling through the generic Pydantic fallback
• preserve integer-backed enums as INTEGER schemas while serializing enum values in the string form expected by the SDK schema model
• add a FunctionDeclaration.from_callable regression test for IntEnum parameters

Testing

• python3 -m py_compile google/genai/_automatic_function_calling_util.py google/genai/tests/types/test_types.py
• python3 -m pytest google/genai/tests/types/test_types.py -k int_enum_parameter (fails locally during test startup: ModuleNotFoundError: No module named 'google.auth')