Limit QPS in list benchmark to reduce CPU variance

[Jkhall81]

Stabilize list benchmark by limiting QPS

This PR adds an explicit QPS limit to the request-benchmark invocation used by the list benchmark to reduce run-to-run CPU variance introduced by streaming LIST responses.

Changes:

1. Adds a fixed --qps=500 flag to the list benchmark deployment (clusterloader2/testing/list/deployment.yaml)
2. Ensures request rate is paced rather than unbounded when running the LIST benchmark
3. Improves stability and repeatability of CPU usage measurements without changing benchmark semantics

Which issue(s) this PR is related to:
Fixes kubernetes/kubernetes#130869

Special notes for your reviewer:
This change intentionally uses a fixed QPS value to minimize variance with minimal surface area. The specific value is not a throughput target and can be adjusted or parameterized in a follow-up if needed. Streaming behavior and benchmark workload remain unchanged.

Does this PR introduce a user-facing change?

NONE

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Jkhall81
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