CustomContainerTrainingJob.run drops max_wait_duration=0 instead of requesting indefinite DWS wait

Thanks for maintaining this library. `CustomContainerTrainingJob.run(max_wait_duration=0)` drops the value because the high-level training-job implementation uses a truthiness check. This prevents callers from requesting the documented indefinite wait for DWS Flex Start.

#### Environment details

- OS type and version: macOS
- Python version: 3.12
- `google-cloud-aiplatform` versions: reproduced in 1.148.1; source inspection confirms the same behavior in 1.150.0, 1.163.0, and current `main`

#### Steps to reproduce

1. Create a `CustomContainerTrainingJob` using any valid project, staging bucket, and container image.
2. Run it with Flex Start and `max_wait_duration=0`.
3. Inspect the scheduling inputs produced by `_prepare_training_task_inputs_and_output_dir` or the resulting `TrainingPipeline` request.

#### Code example

```python
job.run(
scheduling_strategy=custom_job.Scheduling.Strategy.FLEX_START,
max_wait_duration=0,
)
```

Observed scheduling value:

```text
max_wait_duration: None
```

Expected scheduling value:

```text
max_wait_duration: 0s
```

For comparison, a positive input such as `14400` is preserved as `14400s`.

#### Cause

The high-level training-job path converts the duration using a truthiness check, so zero follows the same branch as `None`:

https://github.com/googleapis/python-aiplatform/blob/v1.163.0/google/cloud/aiplatform/training_jobs.py#L1663-L1667

The generated API contract says that explicit zero means indefinite waiting, while omission defaults to 24 hours:

https://github.com/googleapis/python-aiplatform/blob/v1.163.0/google/cloud/aiplatform_v1/types/custom_job.py#L577-L582

`max_wait_duration` is a presence-aware `google.protobuf.Duration`, so explicit zero and an omitted field are distinct requests.

A previous fix correctly changed the lower-level `CustomJob` and hyperparameter-tuning paths to use `is not None`, explicitly noting that `0` is valid:

https://github.com/googleapis/python-aiplatform/commit/d9675fdf051233539f478187143f2833fd6e6af0

That commit did not update the `CustomContainerTrainingJob` path in `training_jobs.py`.

#### Suggested fix

Use an explicit `is not None` check in `_prepare_training_task_inputs_and_output_dir`, consistent with the lower-level implementation, and add tests covering omitted, zero, and positive durations.

#### Stack trace

No exception is raised. The problem is a silent request-semantics change: `0` is serialized as omission.

Start in google/cloud/aiplatform/training_jobs.py at _prepare_training_task_inputs_and_output_dir, then compare the lower-level CustomJob implementation referenced in the issue. Verify the scheduling inputs for omitted, zero, and positive max_wait_duration values; done means zero is serialized as 0s while omission remains distinct and positive durations are preserved.

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