Stabilize workflow parallelism test assertion

[zebbern]

Summary

• replace fragile sleep-duration lower bounds in the workflow parallelism test
• assert branch overlap, merge ordering, and faster-than-serial runtime instead
• keeps the test focused on scheduler parallelism rather than setTimeout precision

Verification

• bun --cwd worker test src/temporal/tests/workflow-runner.test.ts
• 20 focused runs of the parallelism test
• bun run typecheck
• bun run lint
• bun run test
• git push pre-push hook reran typecheck and tests successfully

[Copilot]

Pull request overview

This PR stabilizes the “workflow parallelism” unit test in the Temporal workflow runner by replacing fragile timer-duration lower-bound assertions with assertions that focus on scheduler behavior (branch overlap, merge ordering, and faster-than-serial runtime).

Changes:

• Replace per-branch elapsed-time lower-bound assertions with a branch-overlap assertion.
• Add an assertion that merge starts after both branches complete, and relax the total-runtime upper bound to reflect a serial baseline.

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