[Perf] Optimize and fix informer leak in core/bench benchmark tests

[tetianakh]

What type of PR is this?

/kind bug
/kind regression

What this PR does / why we need it:

Speed up benchmarks:

1. Introduce performance-optimized AddNodeDirect and AddPodDirect methods in fake Kubernetes client helper that bypass standard API-like Create serialization, deep-copy, and reactor overhead by writing directly to the object tracker.
2. Leave existing AddNode and AddPod methods unchanged to ensure zero impact on existing integration tests.
3. Pass a localized cancellable context to the Autoscaler builder per benchmark iteration and cancel it at the end of the iteration, resolving informer goroutine and cache leaks.

date && go test -v -bench=RunOnce -run=^$ -benchmem -count=6 -benchtime=1x ./core/bench/ > after-core.txt && date
Tue May 19 02:59:20 PM CEST 2026
Tue May 19 02:59:40 PM CEST 2026

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

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[k8s-ci-robot]

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: tetianakh
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[k8s-ci-robot]

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[k8s-ci-robot]

PR needs rebase.

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