feat: re-export BrokerUsecase from top-level package

[SarthakB11]

Description

Re-export BrokerUsecase from the top-level faststream package so users who want to subclass it for a custom broker can rely on a stable import path. Previously it was only reachable via faststream._internal.broker.broker, which has no stability guarantees across non-breaking releases.

from faststream import BrokerUsecase

Minimal subset of #2806. Other subclassing types (MsgType, ConnectionType, BrokerConfigType, Registrator) and a custom-broker docs page are left as separate follow-ups.

Fixes #2806

Type of change

• New feature (a non-breaking change that adds functionality)

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
• I have made the necessary changes to the documentation — N/A, no public-API surface change beyond the re-export itself
• My changes do not generate any new warnings (mypy faststream clean across 496 files; bandit -ll no medium/high)
• I have added tests to validate the effectiveness of my fix or the functionality of my new feature — N/A, pure re-export with no behavior change. Smoke-checked locally: python -c "from faststream import BrokerUsecase; import faststream; assert 'BrokerUsecase' in faststream.__all__".
• Both new and existing unit tests pass successfully on my local environment — ran the typing-dependent suite locally (pytest tests/docs/getting_started/subscription/test_annotated.py → 10 passed). Skipped full just test-coverage (heavy) and broker-integration tests (require Kafka/Rabbit/Redis/NATS infra).
• I have ensured that static analysis tests are passing — ruff format --check, ruff check, and mypy faststream (496 files) all clean.
• I have included code examples to illustrate the modifications