[enhancement] Support indeterminate/unknown state for progress

[Weicao-CatilGrass]

Currently progress values must be between 0.0 and 1.0. We need to support indeterminate/unknown progress.

Modify the progress system so that:

• update_progress("task", -1.0) sets the task to "indeterminate" state
• The internal state should distinguish between 0.0 progress and "indeterminate"
• add_progress and reduce_progress should handle indeterminate state appropriately (e.g., convert to 0.0 first)

Use -1.0 as the special value to represent unknown progress in the public API.