`X-docker-isolation-provider`s execute handlers not concurrently

This is easily seen in a simple test [rust-tets](https://www.npmjs.com/package/@konard/rust-tests).
A new handler sends only after the past:

Start with the linked @konard/rust-tests reproduction and trace how X-docker-isolation-provider handlers are dispatched. Confirm the current sequential behavior, then identify the handler execution entry point and add or update a test showing that handlers run concurrently; done means the reproduction demonstrates concurrent execution.

Written by the indexing model from the issue text.