feat(sdks): enhance Kotlin SandboxPool readiness and observability

[ninan-nn]

Summary

• refine the experimental Kotlin SandboxPool configuration so acquire and warmup paths can use different readiness settings
• add warmupSandboxPreparer to initialize a warmed sandbox after readiness succeeds and before it is put into the idle pool
• expand pool observability with detailed lifecycle state, idle entry snapshots, maxIdle/failure/backoff/in-flight counters, and state store idle entry inspection
• extend PoolCreationSpec with extensions and builder conveniences so pool-created sandboxes align better with Sandbox.Builder
• update Kotlin SDK README examples for the new pool capabilities

Testing

• Not run (explain why)
• Unit tests
• Integration tests
• e2e / manual verification
• Ran ./gradlew :sandbox:test --tests *Pool*

Breaking Changes

• None
• Yes (describe impact and migration path)
• This change targets the experimental Kotlin sandbox pool API surface.

Checklist

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

[chatgpt-codex-connector]

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[Pangjiping]

LGTM