[Presets] Typed models and stored format overhaul

[peterschmidt85]

The presets implementation relied on untyped dicts passed between components, unlike the rest of dstack. This PR refactors presets to use pydantic models where possible, making a common model layer across all preset components: the stored preset, the agent report, the Claude stream events, and the session state.

Backward compatibility

• Presets and sessions created by 0.21.x remain readable: they are upgraded on read and rewritten in the new format on save (compatibility code to be removed in 0.22).
• Presets created before 0.21 cannot be read; dstack preset suggests deleting or recreating them.

🤖 Generated with Claude Code