fix: convert Union[Pydantic, Pydantic] tool args at runtime

FunctionTool._preprocess_args only converted dict args to a Pydantic
model for single-model and Optional[Model] annotations. A
Union[ModelA, ModelB] parameter was left as a raw dict, so
isinstance checks inside the tool failed with "Unexpected entity
type: <class 'dict'>"

Use pydantic.TypeAdapter to validate against the full Union so
pydantic picks the matching member. None and instances of any
declared union member pass through unchanged; instances of
unrelated BaseModels fall back to the existing graceful-failure
warning path.

Close #5799

Change-Id: Ie69f8efc8395162eac375a0eaad0c77ed2097cec