Prior to this fix, `_TaskRunner.run` would immediately cleanup the
keep-alive port once the task completed, which would result in the
isolate shutting down as soon as the task result was returned from
`ext.cocoonRunTask` callback in the form of a
`ServiceExtensionResponse`. Since the service extension response is
processed by the service isolate, it was possible for the VM to start
shutting down before the service isolate could send the task result data
back to the task runner.
This change introduces a new service extension,
`ext.cocoonTaskResultReceived`, that the task runner invokes after it
receives the task result from `ext.cocoonRunTask`, notifying the task
process that it can close the keep-alive port and shutdown.
Fixes https://github.com/flutter/flutter/issues/155475
