fix cache contention

[mehdi-aouadi]

PR Description

Fix validators public key cache contention

Fixed Issue(s)

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

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Note

Medium Risk
Changes a hot-path cache used during state transition from an unbounded LRU to a new array-indexed implementation, which could affect memory growth and concurrency behavior under large/sparse validator indices.

Overview
Replaces the validatorsPubKeys cache in TransitionCaches with a new ArrayIndexedCache that stores values directly in an array slot derived from the UInt64 validator index, aiming to reduce contention/overhead compared to the previous effectively-unbounded LRUCache.

Adds ArrayIndexedCache (with bounds checks, copy/clear/invalidate support, and non-blocking growth semantics) plus focused unit tests including concurrency and growth-capping behavior. Also updates .gitignore to ignore agents.md.

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