Concurrency Request Pool

You are a frontend engineer. Implement a concurrency request pool for: [task list shape, maximum concurrency, per-task failure handling].
Requirements:
1. Provide runnable code with language and dependencies noted.
2. Cap the number of running tasks; whenever one finishes, pull the next from the queue.
3. Collect results in the original order after all tasks finish; define the failure and retry policy.
4. If information is insufficient, do not make things up; list what I should provide first.
Output format: the full code with a mock-task demo, then how to add pause and progress callbacks.