The `str | None` syntax is only supported in Python 3.10+ and we can
support more Python versions without compromising readability by
importing the `Optional` type.
This commit makes the following changes:
- Adds a model "Echo" for the request body
- Changes the main endpoint from /create to /echo (more REST-y)
- Sends "400: Bad Request" for invalid params or a reserved path
- Sends "409: Conflict" when trying to use an already registered path
- Prints the server port to stdout
- Removes unnecessary subcommands/options (start, stop, --background)
This allows us to simulate HTTP responses from Browser JS tests.
Instead of using hacks like data URLs to "load" external data,
we can now generate an actual HTTP response that contains
arbitrary headers, body, and has a defined response delay.
