[Lancetnik]

Stacked on #3113. Tickets 03–06 of the Call assertions plan, one broker per commit.

#3113 gave Kafka handlers and publishers key and partition as named arguments of the three Call assertions. The other four brokers now take their own fields the same way, under the names their publish() uses:

Broker	Fields
RabbitMQ	exchange, routing_key, message_id, priority
NATS	subject
Redis	channel, list, stream
MQTT	topic, qos, retain

await handle.assert_called_once_with(
    Data(name="John", user_id=1),
    routing_key="test-queue",
)

Every field compares exactly, as the client delivered it. A batch refuses them the way it refuses headers, and a field asked of a message another broker delivered raises SetupError naming the broker.

What changes

A preparatory commit moves field_reader from _internal/kafka to _internal/testing/calls.py, beside BrokerFields, and gives it the broker's name as a parameter. The guard and its refusal text were Kafka's; now every wrapper shares them, and a broker is a wrapper class and a read callable.

Each broker commit adds faststream/<broker>/call_wrapper.py with the <Broker>CallAssertions mixin and the <Broker>HandlerCallWrapper, sets _call_wrapper_class on the subscriber base and the publisher, and narrows their __call__ to the wrapper, as Kafka does. The classes are internal and not exported.

• RabbitMQ reads the four fields off the aio_pika IncomingMessage with getattr; the in-memory PatchedMessage is a subclass, so the guard holds under TestRabbitBroker.
• NATS answers subject for a core or JetStream message; a key-value or object-store entry has none, so subject= on one raises SetupError naming the kind. The attribute and the overloads sit once on LogicSubscriber, inherited by all eight variants.
• Redis keeps the user's three names and maps them inside the reader: the raw dict stores every address under channel, and its type says the kind. Naming the wrong kind (list= on a pub/sub message) raises SetupError saying which kind the message is and which name answers for it.
• MQTT reads topic, qos and retain off the zmqtt message. The max_workers overloads on MQTTRegistrator.subscriber this needs landed in fix(mqtt): subscriber() is typed by max_workers, so a decorated handler stays typed #3123.

Neither RabbitMQ nor MQTT has batch consumption, so those two have no batch refusal test; the generic guard stays covered by Kafka and Redis.

Tests and docs

• Each broker's test_test_client.py: the three methods with the fields on a handler and on a publisher, the mismatch text naming the field, the batch refusal where the broker has batches, the wrong-kind refusal (NATS stores, Redis kinds), and a handler shared with another broker refusing the field for that broker's message.
• Each broker's tests/mypy/<broker>.py pins a decorated handler and a published handler as the broker's wrapper and calls the three methods on a publisher.
• Each broker's advanced_testing.py snippet gains a test_<broker>_fields, the docs runner calls it, and the "Validates Message Fields" section names every broker's fields with a tab per broker.

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