test(mqtt): fix env var race in topic_writer_tests

[lxsaah]

Description

Fixes the intermittent test_topic_resolver_from_env failure that broke the Comprehensive Development Check job in CI run #828 on main:

panicked at aimdb-mqtt-connector/tests/topic_writer_tests.rs:212:5
  left: None
 right: Some("env/dynamic/topic")

Root cause: test_topic_resolver_with_connector_registration also set and removed TEST_MQTT_TOPIC, but never read it. Its resolver reads TEST_COMMAND_TOPIC. Cargo runs tests in parallel, so its remove_var could land between the other test's set_var and var calls.

Fix: remove the unused set_var/remove_var so TEST_MQTT_TOPIC is only used by one test. The other env-mutating tests (TEST_INBOUND_TOPIC, KNX KNX_SWITCH_GROUP/KNX_SWITCH_INPUT) each use their own variable, so they don't have this problem.

Related Issue

• N/A (CI failure on main after Zero-allocation connector boundary (design 054) #296)

Checklist

• My code follows the project's coding standards (cargo fmt --check clean).
• All existing tests in the affected file passed: cargo test -p aimdb-mqtt-connector --features "std,tokio-rustls" --test topic_writer_tests (17/17, run 5×). Full make check not run locally.
• I have added tests to cover my changes. (Not applicable: test-only fix.)
• I have updated the documentation accordingly. (Not applicable.)

🤖 Generated with Claude Code

https://claude.ai/code/session_01BcQmgAzd8UMfUWiAFN4swa

---

Generated by Claude Code