fix: tolerate unsupported task variants

[Shedna]

Description

• platform GET /tasks and GET /tasks/:id responses can include task variants the Python SDK does not actively model yet
• add an UnsupportedTask model so list_tasks() and get_task() preserve those responses instead of failing during parsing
• keep task creation and task-type filtering limited to the existing stable SDK task types

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.