fix: prevent race condition in idle worker cleanup

[alan-agius4]

Resolves an issue where workers could be terminated while still in use due to an ordering problem between increasing worker usage and clearing the idle timeout. This caused AssertionError: 1 !== 0 during cleanup.

Closes #816

[metcoder95]

can you add a test for it?
Just to facilitate detect regressions

[alan-agius4]

@metcoder95 I considered adding a test, but I'm not sure how to detect this specific case to ensure the timeout gets cleared earlier. Happy to include it if you have any suggestions!

[metcoder95]

Is a bit harder to get a good reproduction; here i'm assuming the problem roots to a high event loop lag that delay the execution of the timer callbacks leading to a race condition between the execution and the new task being posted.

[Tallyb]

still getting it (building storybook with angular):

AssertionError [ERR_ASSERTION]: Expected values to be strictly equal:
1 !== 0
    at Timeout._onTimeout (.../node_modules/piscina/dist/index.js:317:39)
    at listOnTimeout (node:internal/timers:588:17)
    at process.processTimers (node:internal/timers:523:7) {
  generatedMessage: true,
  code: 'ERR_ASSERTION',
  actual: 1,
  expected: 0,
  operator: 'strictEqual'
}
Node.js v22.16.0