tests: reduce background task timing flakiness - #37

Summary

• relax the background-task status wait window in runtime.test.mjs
• allow a longer window for the brokered cancellation test to observe an in-flight task
• widen the fake interruptible task window so cancel assertions race against a real running task, not normal completion

Why

Closes #33.

The background-task runtime tests are too timing-sensitive on slower machines. Normal startup variance can leave a job in running long enough to fail assertions even when the implementation is correct.

Validation

• manually reproduced the background task completion flow and verified the job reaches completed
• manually reproduced the brokered cancellation flow and verified turnInterrupted: true plus the recorded interrupt in the fake Codex state