[M5] Surface AI provider metadata in task status and internal workbench

[weiuou]

Goal

Expose AI provider behavior to internal testers without adding end-user model selection or billing controls.

Scope

• Include provider metadata in task status artifact metadata and next actions.
• Show model_provider, generation duration, normalization status, and fallback-used state in the workbench.
• Distinguish AI provider failure from paperability failure in next-action guidance.
• Do not expose user-selectable providers in the public UI.

Acceptance Criteria

• Completed tasks visibly show whether AI or procedural fallback produced the base mesh.
• Failed or fallback tasks show clear internal debugging guidance.
• Frontend typecheck passes and existing demo flow remains usable.