feat(streams): Add a fake Kafka-less source for profiling pipelines

[fpacifici]

Adds a profiling mode that runs a pipeline end to end without a Kafka broker. A new FakeSource primitive synthesises messages of random bytes at a configurable rate and terminates after a fixed count, so the cost of the pipeline machinery itself (Python/Rust step execution) can be measured in isolation.

pipeline = fake_streaming_source(
    name="fake_source",
    message_size_bytes=1024,
    messages_per_second=1000.0,
    num_messages=10000,
)

The idea is to instantiate the Arroyo processor with a fake consumer class instead of making the processor create its own consumer.
This can be extended into other types of sources including mock consumers for unit tests without kafka.

The fake source, right now, is pretty bare bone, it does not do anything regarding assignment, partitioning and rebalancing. This is ok as this is meant to profile the pipelines under steady load.

Dependency note for review

We were picking up the latest version of arroyo, though a breaking change was introduced in getsentry/arroyo#540, so we need to limit the version and adapt to the change in another PR.

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Fake consumer ignores pause

Medium Severity

FakeConsumer treats pause, resume, and paused as no-ops, so it can still return new messages after Arroyo applies backpressure via MessageRejected. Arroyo expects a paused consumer’s poll to return None; violating that can break or crash the processor under load, which is exactly when profiling pipelines tend to hit backpressure.

 

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