feat(k8s): add Pool rolling update maxUnavailable, close #610

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Summary

• What is changing and why?

Core Feature:

• Added UpdateStrategy field to Pool CRD with maxUnavailable support, enabling controlled rolling updates of sandbox pods
• Introduced PoolUpdateStrategy interface with recreateUpdateStrategy implementation that handles revision-based pod replacements

Controller Refactoring:

• Restructured reconcilePool into 6 clear phases: eviction handling → sandbox scheduling → upgrade computation → scaling → status update
• Encapsulated return values into ScheduleResult and UpdateResult structs for cleaner data flow
• Merged handlePodEvictions and filterEvictingPods into unified handleEviction function

Update Strategy Logic:

• Implements stable sorting for deterministic pod deletion order (unassigned < assigned, pending < running, not-ready < ready, etc.)
• Calculates unavailable budget excluding already-unavailable pods to avoid unnecessary throttling
• Tracks upgrade progress via new Updated field in PoolStatus

API Changes:

• Added UpdateStrategy type with MaxUnavailable field (intstr for percentage or absolute values)
• Added Updated field to PoolStatus indicating pods running target revision
• Added kubectl print columns for UPDATED count and AGE

Testing:

• Added comprehensive E2E test covering: allocated pod preservation, new allocation during upgrade, maxUnavailable enforcement, status tracking, and eventual completion
• Added ComparePodsForDeletion utility with priority-based ordering for consistent pod selection

Bug Fixes:

• Fixed requeue logic for pending unallocated BatchSandboxes
• Removed redundant idle pod offset logic from scalePool

Why
Previously, Pool template changes triggered immediate recreation of all idle pods without rate limiting, potentially causing service disruption. This change enables safe,
gradual rollouts with configurable availability guarantees—critical for production workloads requiring controlled deployment strategies.

Testing

• Not run (explain why)
• Unit tests
• Integration tests
• e2e / manual verification

Breaking Changes

• None
• Yes (describe impact and migration path)

Checklist

• Linked Issue or clearly described motivation
• Added/updated docs (if needed)
• Added/updated tests (if needed)
• Security impact considered
• Backward compatibility considered

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Codex Review: Didn't find any major issues. Breezy!

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[Generalwin]

LGTM