flaky unit test: child_alarm_with_shuffled_entity_registration

[reubenmiller]

Describe the bug

A PR check run found a flaky unit test regarding the ordering of child messages. Below shows the error output:

FAIL [   8.468s] ( 502/2497) c8y_mapper_ext shuffled_tests::child_alarm_with_shuffled_entity_registration
  stdout ───

    running 1 test
    test shuffled_tests::child_alarm_with_shuffled_entity_registration ... FAILED

    failures:

    failures:
        shuffled_tests::child_alarm_with_shuffled_entity_registration

    test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 296 filtered out; finished in 8.46s
    
  stderr ───

    thread 'shuffled_tests::child_alarm_with_shuffled_entity_registration' (94207) panicked at crates/extensions/tedge_mqtt_ext/src/test_helpers.rs:50:5:
    Payload assertion failed.
     Actual: "303,temperature_high,Temperature high,2026-04-01T06:32:50.496492327Z" 
    Expected message with topic: c8y/s/us/test-device:device:external_sensor, payload: 303,temperature_high,Still high
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
    proptest: Saving this and future failures in /home/runner/work/thin-edge.io/thin-edge.io/crates/extensions/c8y_mapper_ext/proptest-regressions/shuffled_tests.txt
    proptest: If this test was run on a CI system, you may wish to add the following line to your copy of the file.
    cc edb57ff902f26a1738fdceaf0d22fe7d8d4e910e1da4054944e73bd1a0867b58

    thread 'shuffled_tests::child_alarm_with_shuffled_entity_registration' (94207) panicked at crates/extensions/c8y_mapper_ext/src/shuffled_tests.rs:349:1:
    Test failed: Payload assertion failed.
     Actual: "303,temperature_high,Temperature high,2026-04-01T06:32:50.496492327Z" 
    Expected message with topic: c8y/s/us/test-device:device:external_sensor, payload: 303,temperature_high,Still high.
    minimal failing input: ChildAlarmWithShuffledEntityRegistrationArgs = ChildAlarmWithShuffledEntityRegistrationArgs {
        field0: 15668371114638989990,
    }

To Reproduce

Expected behavior

Screenshots

Environment (please complete the following information):

• OS [incl. version]: any
• Hardware [incl. revision]: any
• System-Architecture [e.g. result of "uname -a"]: any
• thin-edge.io version [e.g. 0.1.0]: >= 1.7.1 on a PR

Additional context