Replace .at(-1) with array[length-1] for ES2022 compatibility

[dcroote]

Closes #186

This was an easy one so gave it to Copilot and it looks good (original PR in my fork here but because I will delete the fork, copied below):

CI failing on TypeScript compilation due to Array.at() method usage, which requires ES2022 lib support.

Changes

• Replaced attemptTimeoutMs.at(-1) with attemptTimeoutMs[attemptTimeoutMs.length - 1] in src/index.ts:169

Before:

currentAttemptTimeoutMs = attemptTimeoutMs[i] || attemptTimeoutMs.at(-1);

After:

currentAttemptTimeoutMs = attemptTimeoutMs[i] || attemptTimeoutMs[attemptTimeoutMs.length - 1];

Both expressions access the last array element. The latter works with ES5+ without requiring lib configuration changes.