Keep ready study work moving around unresolved model calls

[mikamikasuki]

Summary

• Preserve unresolved external model requests for reconciliation without treating them as failures.
• Allow independent ready work to use only separately observed successful model concurrency, bounded by the configured maximum.
• Record when measured spare capacity is used; do not duplicate or alter the unresolved request.

Validation

• ests/test_study_scheduler.py
• ests/test_study_analysis_prerequisites.py
• AIDE context interning regression tests

All focused tests passed locally.