fix(priority): correct session desired

[jinzhejz]

• Cap Σ ssn_desired ≤ cluster total_slots inside PriorityPlugin.setup
• Sort open sessions by (priority desc, creation_time asc) before distribution
• Greedy single-pass grant: ssn_desired = min(compute_demand, remaining)
• ssn_allocated runtime update via lifecycle callbacks unchanged

[gemini-code-assist]

Code Review

This pull request implements priority-aware resource distribution within the PriorityPlugin. The core change involves calculating the total available cluster slots during the setup() phase and distributing them among open sessions based on priority (descending) and creation time (ascending) as a tiebreaker. This logic populates a new ssn_desired map, which is_underused now consults to ensure high-priority sessions are prioritized for resource allocation until their calculated share is met. By moving the cluster-capacity cap into the PriorityPlugin, the implementation prevents the aggregate over-allocation issues previously seen in the FairShare plugin. Feedback from the review pointed out a minor typo in the design documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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