[vishalmore90]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Fixes an inconsistency between PodGroup conditions, PodGroup warning events, and Pod status conditions when a gang scheduling restriction cannot be satisfied.

Prior to this PR:

1. gang.OnSessionClose calculated the unmet task count using minAvailable - schedulableTaskNum() when recording the PodGroup Unschedulable condition.
2. SchedulerCache.RecordJobStatusEvent recalculated the PodGroup warning event message using len(job.TaskStatusIndex[Pending]), leading to different task counts being reported for the event vs the condition.
3. Pending pods without explicit predicate errors fell back to job.FitError(), omitting the N/M tasks in gang unschedulable: prefix specified in docs/design/scheduling-reason.md.

This PR:

• Adds GetPodGroupCondition and SchedulableTaskNum to JobInfo to share the calculation logic.
• Updates SchedulerCache.RecordJobStatusEvent to reuse the PodGroup unschedulable condition message for both the PodGroup warning event and pending pod fallback condition.
• Falls back to job.MinAvailable - job.SchedulableTaskNum() when computing gang unready task count if the condition message is not pre-populated.
• Adds comprehensive table-driven unit tests in pkg/scheduler/api and pkg/scheduler/cache.

Which issue(s) this PR fixes:

Fixes #6200

Special notes for your reviewer:

AI disclosure: Prepared with assistance from AI coding tools adhering to Volcano's AI guidance in CONTRIBUTING.md. All code, tests, and verifications were reviewed and verified against repo guidelines.

Does this PR introduce a user-facing change?

Fix inconsistent unschedulable task counts across PodGroup condition, PodGroup warning event, and Pod conditions for gang jobs.

[volcano-sh-bot]

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[vishalmore90]

/review

[JesseStutler]

Thanks. I will review it recently, but your ut failed, please check the CI's log @vishalmore90

[vishalmore90]

@JesseStutler Please Take A Look whenever u get chance