Method: run(fn)

Executes an asynchronous function within the concurrency queue.

Parameters:

• fn – An async function (or function returning a promise) to execute.

Returns:

A promise resolving to the result of fn.

Behavior:

1. If the number of currently running operations is below limit, fn executes immediately.
2. If the limit is reached, the function waits in a queue until a slot is available.
3. After fn finishes, the next function in the queue starts automatically.