[codex] stabilize pool scaling

[GodBlf]

Summary

• synchronize the Helm Pool CRD with PoolStatus.Updated
• make pool buffer calculations use Ready, unallocated pods while retaining Pending pods as in-flight capacity
• bound scale-down batches with scaleStrategy.maxUnavailable and wait for deletions to finish
• delete least-useful idle pods first and preserve stable Ready capacity
• count terminating pods against PoolMax
• keep status updates running while scale expectations are pending

Root cause

Six behaviors amplified one another under slow-starting runtimes: the Helm CRD pruned status.updated, buffer calculations treated Pending pods as available, the buffer target jumped to its midpoint outside the configured band, scale-down was unbounded, terminating pods disappeared from occupancy accounting, and scale-down selected the oldest Ready pods first. Together these caused a hot reconcile loop and sustained pod create/delete oscillation.

Impact

The controller now converges without repeatedly replacing usable pool capacity. Existing Pool manifests remain compatible: no CRD fields, CLI flags, or Helm values were added. The existing scaleStrategy.maxUnavailable setting now limits both scale-up and scale-down batch sizes; the default remains 25%.

Validation

• KUBEBUILDER_ASSETS="..." go test ./internal/controller -count=1 (36 envtest specs)
• targeted pool scaling stability unit tests
• go test ./internal/utils/expectations/... -count=1
• go test ./... -run '^$'
• go vet ./internal/controller ./internal/utils/expectations/...
• helm lint charts/opensandbox-controller
• rendered Helm CRD contains .status.updated and the UPDATED printer column
• pnpm docs:build
• git diff --check

Closes #1423

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[Pangjiping]

LGTM. @fengcone @Spground ptal