test(relay): mesh_demo echo test flaky under parallel suite — passes standalone, fails full-suite on clean main

[brocoppler]

api::mesh_demo::tests::demo_join_forwarded_arm_round_trips_echo fails under the full parallel suite but passes standalone, on clean main (7e34bee), reproduced twice each way:

# fails
cargo test -p buzz-relay --lib
# test api::mesh_demo::tests::demo_join_forwarded_arm_round_trips_echo ... FAILED
# test result: FAILED. 719 passed; 1 failed; 27 ignored

# passes
cargo test -p buzz-relay --lib demo_join_forwarded_arm_round_trips_echo
# test result: ok. 1 passed; 746 filtered out

Environment: macOS (Darwin 25.3.0, M-series), default test parallelism. Behavior is consistent across runs and across two checkouts (main and a feature branch with no relay changes), so it looks like inter-test resource contention (port/state shared with another test in the binary) rather than machine-specific breakage. Same family as #2369.

Not blocking anything on my side — filing since it surfaces as a false red in any full local cargo test -p buzz-relay --lib run.

[Chessing234]

Fixed in #2498 — mesh_demo echo tests take a shared lock and clear Redis lease keys.