feat: introduce processing rate to max ratio as poll-idle-ratio replacement

[Fly-Style]

Summary

Kafka's poll-idle-ratio is not the best possible metric to represent the idle cost for autoscaler - it reflects time spent polling, not whether the task has spare processing capacity. Also, it is not supported for Kinesis, which is still in play in read-world productions.

This patch adds an alternative idle signal : 1 - (avgProcessingRate / maxObservedRate), gated behind a new opt-in flag useUtilizationRatio (default false, so existing deployments are unaffected).

• CostBasedAutoScaler: tracks a bounded watermark of the task's best-observed processing rate (maxObservedRate), feeding CostMetrics.
• WeightedCostFunction: when the flag is on, derives idle ratio from that utilization ratio instead of pollIdleRatio; falls back to IDEAL_IDLE_RATIO until a watermark sample exists (cold start).
• CostBasedAutoScalerConfig: new useUtilizationRatio boolean, wired through builder/serde/equals/hashCode.
• CostMetrics: new nullable maxObservedRate field; old constructor kept (delegates with null) since three test call sites still use it.

This PR has:

• been self-reviewed.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.

[FrankChen021]

Severity	Findings
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Severity	Findings
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P2	1
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Reviewed 8 of 8 changed files.

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This is an automated review by Codex GPT-5.5

[FrankChen021]

[P2] Shut down the background publisher executor

The new test creates a single-thread ExecutorService and immediately drops it. That executor uses a non-daemon worker and is never shut down, so the embedded-test JVM can stay alive after the test returns; on timeout or failure it can also keep publishing into the topic while cleanup is running. Keep a reference and shut it down/cancel the Future in a finally block or use an existing managed executor.