[Feat]: Add excludeArtifacts bool to GetTask request

### Is your feature request related to a problem? Please describe.

`SendMessage` supports `returnImmediately` flag where clients not using push notifications are supposed to poll the task status. `GetTask` can make the server omit history (historyLength: 0), but there's no way to make a lightweight status fetch with artifacts omitted.

### Describe the solution you'd like

Extend `GetTask` request with `excludeArtifacts` bool.

### Describe alternatives you've considered

`includeArtifacts` for symmetry with `ListTasks`. Would need to have a bad default of `true` for making it a non-breaking change.

### Additional context

_No response_

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