test(core): cover LruCache eviction behavior

[1amKhush]

Problem

src/core/utils/lru-cache.ts is shared by transport, gateway, and payment code but has no focused unit test suite. Small changes to eviction or access ordering can therefore affect several unrelated paths without a direct regression failure.

Scope

Add a co-located src/core/utils/lru-cache.test.ts covering the existing behavior:

• inserting and retrieving entries;
• evicting the least-recently-used entry at capacity;
• refreshing recency after get() and after updating an existing key;
• invoking onEvict with the evicted key and value;
• supporting an empty-string key without exceeding capacity;
• has, delete, clear, size, and entry iteration where useful.

Non-goals

• Do not redesign the cache or make it part of the public SDK API.
• Do not change entry-order semantics without first documenting the intended contract in the PR.
• Keep production changes out of scope unless a test exposes a confirmed defect.

Acceptance criteria

• Tests are deterministic and require no timers, relay, or network.
• Tests are co-located and follow the repository's Bun test conventions.
• Existing tests, lint, and type checking continue to pass.

Verification

bun test src/core/utils/lru-cache.test.ts
bun lint
bun typecheck