fix(simulation): stop Step2 hanging after prepare failures (#335)

[Yaohua-Leo]

Summary

Fixes #335 by making Step2 stop on terminal prepare failures instead of hanging in config polling.

Changes

• fail the prepare task when entity filtering yields zero usable entities, so task status and simulation state stay aligned
• expose status, error, and profiles_generated in realtime simulation endpoints used by Step2
• stop frontend polling immediately when realtime APIs report a failed state
• infer coarse simulation entity types for graph nodes that have empty labels, reducing false zero-entity failures

Testing

• GET /api/simulation/entities/mirofish_237fddbbfcd44c23 now returns filtered_count: 20 for a graph whose nodes had empty labels
• created sim_fde532d5b129 and ran /api/simulation/prepare with entity_types=[DefinitelyMissingType]; /api/simulation/prepare/status now returns status: failed with the backend error instead of leaving Step2 in a fake-complete state
• verified GET /api/simulation/sim_0db482d5074d/config/realtime returns status: failed, generation_stage: failed, and the real error message for the previously stuck case

Impact

• minimal scope, only Step2 failure handling and simulation entity selection
• no public breaking API changes; realtime endpoints only add fields consumed by the frontend

Fixes #335