fix: handle unsupported task types in task endpoints

[Shedna]

Description

• the platform /tasks endpoints can now return the beta extract task type, while the Python SDK only supports stable search, fetch, and research tasks
• synchronized the SDK by turning unsupported task types into explicit UnknownError failures in get_task, and by refining TASK_TYPE_NOT_SUPPORTED 403 responses away from misleading authentication errors
• added focused regression tests for unsupported task payloads and 403 error refinement

Checklist

• I have installed prek on this project (for instance with the make install-dev command) before creating any commit, or I have run successfully the make lint command on my changes.
• I have run successfully the make typecheck test command on my changes.
• I have updated the README.md if my changes affected it.