Forward group_instance_id from the FastAPI Kafka router

[00yhj22-debug]

Description

Follow-up to #2871. The FastAPI KafkaRouter accepts group_instance_id in its subscriber() overload signatures, but the parameter was not included in the super().subscriber() call in faststream/kafka/fastapi/fastapi.py. As a result the value was silently dropped before reaching the underlying broker subscriber and the consumer never saw it — calling router.subscriber("topic", group_instance_id="x") on the FastAPI router had no effect.

This PR forwards group_instance_id explicitly in the super() call. The fix is a single line. I also added a fastapi-flavored regression test alongside the existing broker and KafkaRouter cases in tests/brokers/kafka/test_group_instance_id.py.

Type of change

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

Checklist

• My code adheres to the style guidelines (ruff check and ruff format are clean)
• I have conducted a self-review of my own code
• I have made the necessary changes to the documentation (no docs change needed — the parameter is already documented in the FastAPI router`s docstring; it just was not forwarded)
• My changes do not generate any new warnings
• I have added tests to validate the effectiveness of my fix
• Both new and existing unit tests pass successfully on my local environment (pytest tests/brokers/kafka/test_group_instance_id.py — all five tests pass)