[dt serve] Support running in machine mode and when `--run-app` is true

When the server is started with `--machine`, it uses machine-readable JSON as the output. Using`--run-app` requires reading the `stdout` which is currently not set up to understand JSON.

Support for this should be added if we want to be able to debug a DevTools app when using `--machine` (for example, to debug a DevTools instance embedded in VS Code or intelliJ).

Start at the `dt serve` entry point and trace how `--machine` formats output and how `--run-app` reads stdout. Confirm the expected machine-readable JSON flow for both flags, then verify that a DevTools app can be debugged in that mode, including the VS Code or IntelliJ scenario described.

Written by the indexing model from the issue text.