Fix Bad Gateway and optimize PHP workers

[JanJakes]

Motivation for the change, related issues

This PR fixes Bad Gateway (502) errors that occur when multiple concurrent requests are made to the PHP request handler.

Implementation details

1. The primary PHP instance was not used for enqueued requests. Instead, all enqueued requests needed to wait for the secondary instances, and the primary instance only served new requests directly, never from the queue.

   The fix refactors PHPProcessManager to a pool-based design where all spawned instances (including primary) are added to an idle pool, acquirePHPInstance() takes from the pool or spawns new ones if needed, and instances are returned to the pool for reuse. All PHP instances should be automatically rotated every 400 requests.

2. Increased instance wait timeout from 5s to 30s: When all PHP instances are busy, new requests wait for one to become available. The previous 5s timeout was likely a bit too short for some scenarios. This timeout limits wait time for an available instance, not PHP execution time.

3. Semaphore stale resolver bug: When a semaphore acquire timed out, its resolver remained in the queue. Later releases would notify these stale resolvers instead of actual waiting requests. The fix removes timed-out resolvers from the queue.

Testing Instructions (or ideally a Blueprint)

The fix can be verified by running concurrent requests in the browser console:

const iframe = document.querySelector('iframe');
const playground = iframe.contentWindow.playground;
const start = Date.now();
const requests = [
  playground.run({ code: '<?php sleep(4); echo "done1";' }),
  playground.run({ code: '<?php sleep(4); echo "done2";' }),
  playground.run({ code: '<?php sleep(4); echo "done3";' }),
  playground.run({ code: '<?php sleep(4); echo "done4";' }),
  playground.run({ code: '<?php sleep(4); echo "done5";' }),
  playground.run({ code: '<?php sleep(4); echo "done6";' }),
  // Add more and modify sleep time as needed for testing.
];
const results = await Promise.allSettled(requests);
console.log('Total time:', Date.now() - start, 'ms');
console.log(results);

On trunk, some of these promises would be rejected. In this branch, all should succeed.

[adamziel]

Woah all tests are green ❤️

[adamziel]

good catch!