test: worker children in two more test files should exit with the test process #712

Follow-up to #693 / #705.

Two more test files park worker child processes whose teardown runs only in a finally, so a hard-killed or timed-out test run can leave those workers running:

• packages/cli/test/bridge-ack-path.test.ts (workers waiting on release-… file gates, ~:294 and ~:309)
• packages/cli/test/mail-existing-updates.test.ts (~:150)

The fix: start these workers so they exit when the test process goes away (the parent-pid and stdin watch from the live-process helper in #705, or an equivalent), and prove it with a test that hard-kills the owning process and asserts the workers are gone within a bounded deadline.

Do this after #705 merges, since it reuses that helper.