Added parallel processing of subscribers in EventBroker during asynchronous execution

[mom1]

Summary

Add parallel async execution support for EventBroker subscribers. This significantly improves performance for I/O-bound handlers by running them concurrently.

Core Changes

parallel=True parameter in EventBroker.__init__():

• Async handlers execute concurrently via asyncio.gather()
• Sync handlers run in thread pool via asyncio.to_thread() — don't block event loop
• Full backward compatibility: parallel=False by default

Implementation Details

• Deduplication: handlers matching multiple predicates receive event only once
• Clean error handling: NotImplementedError when no handlers match
• Mixed handlers support: both sync and async in same broker

Tests & Refactoring

• Applied readable tests methodology (Given-When-Then, Mock > nonlocal)
• Unified factories: removed basket fixture, now using BasketFactory
• Replaced EventBrokerFactory class with pytest fixtures
• Coverage: 100% (added missing parallel+sync handler test)

Example Usage

broker = EventBroker(parallel=True)

@broker.instance(MyEvent)
async def handler_a(event): ...

@broker.instance(MyEvent)
async def handler_b(event): ...

# Both handlers run concurrently:
await broker(my_event)  # ~max(duration_a, duration_b) instead of sum

[github-actions]

🎉 This PR is included in version 0.5.0 🎉

The release is available on GitHub release

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