fix(tools): handle Union[ModelA, ModelB] in FunctionTool._preprocess_args

[devteamaegis]

Problem

FunctionTool._preprocess_args silently skips dict-to-Pydantic conversion when a
parameter is typed as Union[ModelA, ModelB] (two or more non-None members).

The existing code only handled Optional[T] — i.e. exactly one non-None
type in the union:

non_none_types = [arg for arg in union_args if arg is not type(None)]
if len(non_none_types) == 1:
    target_type = non_none_types[0]          # ✅ Optional[T] works
# len > 1 → target_type stays as raw Union → issubclass check always False → dict
# passed to the function unchanged → isinstance() in tool body raises TypeError

Reported in #5799. Real-world failure:

def create_entity(entity: Union[UserProfile, CompanyProfile]) -> str:
    if isinstance(entity, UserProfile):      # ❌ entity is still a dict
        ...

Fix

Add an elif len(non_none_types) > 1 branch that, when the value is a dict,
iterates the Pydantic members of the union in declaration order and calls
model_validate() on the first one that accepts the payload — identical strategy
to the existing Optional[T] path.

elif len(non_none_types) > 1 and isinstance(args[param_name], dict):
    if not any(isinstance(args[param_name], t) for t in non_none_types
               if inspect.isclass(t)):
        pydantic_types = [
            t for t in non_none_types
            if inspect.isclass(t) and issubclass(t, pydantic.BaseModel)
        ]
        for candidate_type in pydantic_types:
            try:
                converted_args[param_name] = candidate_type.model_validate(
                    args[param_name]
                )
                break
            except Exception:
                continue
        else:
            logger.warning(...)
    continue

Tests

Five new cases added to tests/unittests/tools/test_function_tool_pydantic.py:

Test	What it checks
test_preprocess_args_union_first_branch_converted	dict → first union member
test_preprocess_args_union_second_branch_converted	dict → second union member
test_preprocess_args_union_already_correct_type_unchanged	existing instance untouched
test_preprocess_args_union_optional_backward_compat	Optional[T] still works
test_run_async_union_end_to_end	both branches via run_async

Fixes #5799

[adk-bot]

Response from ADK Triaging Agent

Hello @devteamaegis, thank you for creating this PR!

This PR is a great fix for the issue with Union types in FunctionTool. To help the reviewers merge your PR more efficiently, please ensure you've addressed the following items from our contribution guidelines:

• Summary of passed pytest results: Please include the console output of your passed unit tests to show that they run successfully in your environment.
• Logs or E2E evidence: Since this is a bug fix, providing some command-line logs or a brief execution log showing the fix successfully handling the Union model validation end-to-end would be highly beneficial for the reviewers.

Thank you for your valuable contribution!