test(runtime): make the Stop/timeout lifecycle race deterministic

Problem

packages/runtime/src/__tests__/shell-run-manager.test.ts intermittently fails in CI at keeps the first committed lifecycle cause across Stop and timeout races.

The observed failure expected cancelled but received timed_out. Rerunning the unchanged job passed, which makes the current test timing-dependent and leaves ambiguity between a product race and a nondeterministic test setup.

Evidence:

• CI run 31258873738 on PR #2472.
• The first test_workspaces attempt failed in the Runtime suite; the failed-job rerun passed without a code change.
• PR #1776 previously fixed a related but different Stop/timeout race.

Expected outcome

• Establish whether the first committed lifecycle cause is violated or the test starts Stop too close to the timeout boundary.
• Replace wall-clock scheduling with a deterministic synchronization seam where practical.
• Preserve the intended lifecycle contract; do not weaken the assertion or add retries.
• Verify the focused race repeatedly and keep the Runtime workspace CI stable.