fix: annotate TimersBroker._subscribers as WeakSet for ty 0.0.87

[lesnik512]

Fixes #106.

The scheduled dependency check picked up ty 0.0.87, which turns on invalid-mutable-override by default and now checks overrides of attributes declared in __init__. It reports five errors on main.

• TimersBroker annotated _subscribers and _publishers as list, but faststream's Registrator sets both to a WeakSet. _subscribers is now annotated as WeakSet[TimersSubscriber], the same as faststream-outbox, with an ignore because WeakSet is invariant. Nothing reads _publishers, so its annotation is gone, along with the import it used.
• _outer_config: TimersBrokerConfig narrows faststream's BrokerConfig in three places. faststream's bases aren't generic over the config, so each line gets a per-line ignore.

Locally with ty 0.0.87, just lint-ci is clean and just test-ci passes (168 passed, 100% coverage).