feat(scheduler): add built-in scenario generators

[enoodle]

Description

Stack slice 5 of 9 for the reclaim generator portfolio.

References:

• Full implementation PR: feat(scheduler): add bounded scenario generator portfolio #1716
• Design: docs(scheduler): add reclaim generator portfolio design #1696 (docs/developer/designs/reclaim-generator-portfolio-design.md)
• Original issue: scale: reclaim failure test first regresses in v0.14.3 #1660

This PR adds the built-in scenario generator implementations:

• Adds NodeLocalGreedy.
• Adds MultiNodeGang.
• Adds generator tests close to the scenario emission behavior.

Review focus for this slice is the generated scenario shape, scenario quality, and accumulated-filter contract. Portfolio driving is introduced in the next PR.

Related Issues

Related to #1660.

Checklist

Note: Ensure your PR title follows the Conventional Commits format (e.g., feat(scheduler): add new feature)

• Self-reviewed
• Added/updated tests (if needed)
• Updated documentation (if needed)

Breaking Changes

None.

Additional Notes

Stacking:

• Base: erez/reclaim-generator-portfolio-04-wire-budgets
• Next PR: erez/reclaim-generator-portfolio-06-portfolio-driver

Verification for the rebuilt stack:

• GOCACHE=/tmp/kai-reclaim-generator-go-cache go test ./pkg/scheduler/plugins/scenariogenerators ./pkg/scheduler/conf_util ./pkg/operator/operands/scheduler ./pkg/scheduler/actions/common/solvers ./pkg/scheduler/actions/reclaim ./pkg/scheduler/actions/preempt ./pkg/scheduler/actions/consolidation ./pkg/scheduler/actions/integration_tests/reclaim ./pkg/scheduler/cache ./pkg/scheduler/cache/status_updater ./pkg/scheduler/metrics ./pkg/apis/scheduling/v2alpha2 -count=1
• git diff --check
• GOCACHE=/tmp/kai-reclaim-generator-go-cache make validate

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