[Swarming] Add backpressure mechanism to SwarmingService

[IvanBM18]

Overview

This PR implements a backpressure mechanism in SwarmingService to prevent overloading the Swarming pool with too many tasks.

Changes

• Added MAX_PENDING_TASKS = 50 to limit the number of pending tasks.
  • This threshold was chosen considering a pool of at most 100 bots and a scheduler running every 2.5 minutes. It ensures bots don't go idle (having at least ~5 tasks in the queue at all time)
• Updated create_utask_main_jobs to check the queue size before pushing each task using Swarming's CountTasks RPC.
• Implemented a Fail Closed strategy: if the CountTasks rpc call fails, we assume the queue is full and stop scheduling further tasks in the batch.
• Added unit tests to verify backpressure and failure handling.

Note

This PR is dependant on this other PR so please go ahead and review that one first :D