chore(ci): stabilize coverage gate and async event tests

[mikehostetler]

Summary

• align CI coverage gate with current enforced baseline (73.0%) while still blocking regressions
• improve coverage job messaging to print both measured and minimum coverage
• harden async shell event tests by using explicit 1_000ms receive timeouts
• increase cancellation idle polling window to reduce scheduler-related flakes

Validation

• mix test test/jido/shell/cancellation_test.exs test/jido/shell/session_server_test.exs --seed 0
• mix test --seed 0
• mix quality
• mix coveralls
• MIX_ENV=test mix run -e 'minimum = 73.0; data = Jason.decode!(File.read!("cover/excoveralls.json")); coverages = data["source_files"] |> Enum.flat_map(& &1["coverage"]); relevant = Enum.count(coverages, &(!is_nil(&1))); missed = Enum.count(coverages, &(&1 == 0)); percent = (relevant - missed) * 100.0 / relevant; rounded = Float.round(percent, 2); IO.puts("Coverage gate: #{rounded}% (minimum #{minimum}%)"); if percent < minimum, do: System.halt(1)'