test(sower): wait for async task in deploy_subscription policy test
The test did not await the spawned Task.Supervisor child, causing the
test process (DB sandbox owner) to exit while the task still held a
connection. Apply the same Process.monitor + assert_receive pattern used
by handle_deployment_request/2 tests.
sow-154
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>