fix(import_devices): reject a device group containing '+'

[TomlDev]

Optional hardening requested in the review of #207, as a separate PR. Refs #206.

Problem

The school-qualified LINBO group id <school>+<group> (#207, #206) is unambiguous only because a group name never contains +. Today only sophomorix guarantees that. linuxmuster-api derives the owning school from the file name, so base7 has to enforce it as well.

Change

validateDeviceRow() rejects a device row whose group (column 3) contains +. The row is skipped like any other invalid row: it returns (False, None), so getDevicesArray() leaves the device out and import-devices does not process it. The check runs before the hostname, MAC and IP checks and silently returns, in the same way as those checks; no new log line is added. The separator is a named constant, LINBO_GROUP_SEPARATOR, in functions/network.py.

validateDeviceRow() receives the plain group from devices.csv (the group column is never stored qualified, see #207), so there is no already-qualified id on this path and every + is rejected, for default-school and for other schools alike.

Tests

tests/test_functions_network.py:

• a valid group is accepted and returned unchanged, for default-school and a non-default school
• groups with + (abc+raum101, raum+101, leading and trailing +) are rejected for both
• the input row is not modified by a rejected row for a non-default school

pytest tests with a stand-in environment module (linuxmuster-common is not installed on my machine): 41 passed (30 before). flake8 (--max-line-length=100) reports nothing new in the changed files.

Base

Based on feat/per-school-groups, where the other per-school group changes go.