scripts/mpos_controller: give the serial port to mpremote - #226

The problem

MPOSController accepts --serial-port for its own REPL connection. But it
started mpremote with no connect argument. mpremote then connects to the
first USB serial device that it finds:

# main.py
def ensure_connected(self):
    if self.transport is None:
        do_connect(self)          # no args, so dev becomes "auto"

# commands.py
elif dev == "auto":
    for p in sorted(serial.tools.list_ports.comports()):
        if p.vid is not None and p.pid is not None:
            state.transport = SerialTransport(p.device, baudrate=115200)
            return

The REPL connection used the correct port, because it uses pyserial directly.
Only the file transfers used the incorrect port.

Why this is difficult to find

If the host has one device, that device is always the correct device. The
problem stays hidden. If the host has two devices, mpremote sends the files
to the incorrect device.

mpremote sorts the devices by name. Because of this, it selects the same
incorrect device each time. A random selection is more safe: it fails only some
of the time, and the user then examines the connection quickly. This selection
fails all of the time. The user sees no error, and sees no change on the
correct device. The user then examines the application code, and does not
examine the connection.

How to see the problem

1. Connect two devices to the host.
2. Give this command:
   python3 scripts/mpos_controller.py --serial-port <the second device> installapp <path>
3. Examine the two devices. The first device has the new files.

The change

Four functions started mpremote without the port:

Because screenshot() and get_widget_tree() also used the incorrect device,
a host with two devices showed the old screen after a correct installation.
This gave more incorrect data to the user.

All four functions now build the command with a new _mpremote_cmd() helper.
The helper puts connect <port> in the command when the port is known.

SerialBackend.run_test_file() is newer code, and it already gives
connect self.port to mpremote. This change makes the four older functions do
the same thing. I did not change run_test_file(), because it is correct.

The installapp action also stops and gives an error message if the user gives
no --serial-port and the host has more than one device. It is not possible to
select the correct device in that condition.

tests/cpython_mpos_controller.py had the same problem. The app management
test received a serial_port argument, but it did not use that argument.

Tests

A new mpremoteport section tests _mpremote_cmd(). It does not use a device,
so it runs with the desktop backend:

$ python3 tests/cpython_mpos_controller.py --only mpremoteport
  [mpremote port selection]
  + with a port: the command contains 'connect'
  + with a port: the port comes after 'connect'
  + with a port: the command keeps the other arguments
  + with no port: the command contains no 'connect'
  + with no port: the command keeps the other arguments

To show that the test is sufficient, I replaced _mpremote_cmd() with the old
behaviour. Two of the five checks then failed.

I also tested the change with one device. The files went to that device.
I did not test with two devices, because I have only one device.

Parts that this change does not correct

scripts/install.sh also starts mpremote with no connect argument. That
script has no port option, so it does not ignore a port that the user gave. It
always uses the first device. To correct it, it must get a new port option.
This is a change to its interface, so it is not in this pull request.

AGENTS.md also shows an mpremote command with no port. It has the same
condition.

Checklist

• CHANGELOG.md: a new Tools: item is in the "Future release" section.
• Tests: a new test section, as shown above.
• make lint: all checks pass.
• Formatting: the new code keeps the style of the code near it. I did not run
  ruff format on these files, because the files were not formatted with it
  before this change. make lint and the CI workflow use ruff check only.
• MANIFEST.JSON and MAINTAINERS.md: no change, because this pull request does
  not change an app or a board.