fix(_utils/_transform): propagate __api_exclude__ in _async_transform_recursive (#3324)

## Summary

The sync `_transform_recursive` path (line 221 of
`_utils/_transform.py`) correctly passes `exclude=getattr(data,
"__api_exclude__", None)` to `model_dump()` when it encounters a
pydantic `BaseModel` value. The async counterpart
`_async_transform_recursive` (line 387) was missing this argument.

## Bug

```python
# sync — correct
if isinstance(data, pydantic.BaseModel):
    return model_dump(data, exclude_unset=True, mode="json", exclude=getattr(data, "__api_exclude__", None))

# async — missing exclude= kwarg (before this fix)
if isinstance(data, pydantic.BaseModel):
    return model_dump(data, exclude_unset=True, mode="json")
```

## Impact

`ParsedResponse` (in `openai/types/responses/parsed_response.py`)
declares:

```python
__api_exclude__ = {"parsed_arguments"}
```

This is specifically to prevent the client-side computed
`parsed_arguments` field from being sent back to the API server in
multi-turn conversations. With the async client that exclusion had no
effect: `parsed_arguments` leaked into every request body that included
a `ParsedResponse` in the message history.

## Fix

One-line change — add the missing `exclude` kwarg to match the sync
path:

```python
return model_dump(data, exclude_unset=True, mode="json", exclude=getattr(data, "__api_exclude__", None))
```

## Tests

Added `tests/test_async_transform_api_exclude.py` with three tests (no
live API calls):

1. `test_async_transform_excludes_api_excluded_fields` — asserts
`__api_exclude__` fields are absent from the async output (this test
**fails before the fix, passes after**)
2. `test_sync_transform_excludes_api_excluded_fields` — sanity-check
that the sync path behaves the same
3. `test_async_transform_pydantic_model_without_api_exclude` — models
without `__api_exclude__` are unaffected

Run with `PYTHONPATH=src pytest
tests/test_async_transform_api_exclude.py`.

---------

Co-authored-by: Marcus Wood <marcuswood@openai.com>