chore(tests): Cover mqtt e2e scenarios for MqttSession

[borisalekseev]

I wrote only e2e tests according to the conversation at #928

[allenporter]

Thank you @borisalekseev for following up on the discussion from #928 and putting together this real-broker E2E test suite! (Note: I am currently experimenting with agentic code review to help review PRs, so please let me know if anything in the feedback looks off).

These tests are well-structured, keep traffic isolated with per-test UUID topics, and effectively exercise the behavioral contracts of both MqttSession and MqttChannel without modifying production dependencies.

I have left a few inline comments on:

1. Ensuring broker tests have bounded timeouts and are tagged so standard local pytest runs stay fast and offline-friendly.
2. Decoupling the EMQX service container into a separate parallel job or standalone workflow.

[allenporter]

We should keep pytest (or uv run pytest) as the standard instruction for general testing, and document Docker Compose specifically under the pytest -m mqtt_broker section for running the real-broker test suite.

[borisalekseev]

Updated the guide: uv run pytest remains the standard command, with Docker Compose documented separately for uv run pytest -m mqtt_broker.

[allenporter]

Looks great, thank you @borisalekseev

One of the tests appears to take 10 seconds, presumably a timeout, so something to look into speeding up in a future PR.

Thanks again.