[Lancetnik]

Description

MQTTRegistrator.subscriber returns MQTTDefaultSubscriber | MQTTConcurrentSubscriber. Strict mypy treats a union-typed decorator as untyped, so any handler under @broker.subscriber(...) on an MQTTBroker or MQTTRouter reports untyped-decorator, and the handler loses its type:

@broker.subscriber("t")
def handle() -> None: ...
# error: Untyped decorator makes function "handle" untyped  [untyped-decorator]

The registrator gains the two max_workers overloads the MQTT FastAPI router already has: max_workers: Literal[1] returns MQTTDefaultSubscriber, any other int returns MQTTConcurrentSubscriber. tests/mypy/mqtt.py pins both subscriber types and the decorated handler's type; both checks go red on the old registrator.

Split out of #3120, where the MQTT Call assertions needed it; it is a fix on its own.

Type of change

• Bug fix (a non-breaking change that resolves an issue)

Checklist

• My code adheres to the style guidelines of this project (just lint shows no errors)
• I have conducted a self-review of my own code
• My changes do not generate any new warnings
• I have added tests to validate the effectiveness of my fix or the functionality of my new feature
• I have ensured that static analysis tests are passing by running just static-analysis

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