Further investigate flaky integration tests and restore concurrent execution

[lawrence-forooghian]

See comments in #73 and the WIP investigation in #74.

Also waiting to hear about the sandbox concurrency limit (https://ably-real-time.slack.com/archives/CURL4U2FP/p1755803879736059).

Update: There seem to be occasional hangs even after switching to running in serial, see e.g. https://github.com/ably/ably-liveobjects-swift-plugin/actions/runs/17155247918/job/48670539579

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