Quick fix VLAN detection for named interfaces

[LambdaLightSource]

PR title

Fix VLAN detection for named interfaces

PR body

Summary

Fix VLAN detection for interfaces whose names do not end directly in a numeric VLAN ID, for example:

p6p3.vlan9

Problem

The current network interface scan assumes that anything after . can be converted directly to an integer VLAN ID:

vlan = None
if len(interface.split(".")) > 1:
    vlan = int(interface.split(".")[1])

This works for interface names such as eth0.9, but raises a ValueError for valid VLAN interface names such as p6p3.vlan9.

Linux exposes the actual VLAN ID in /proc/net/vlan/<interface>, for example:

p6p3.vlan9  VID: 9

Fix

The new VLAN detection helper:

• reads the VLAN ID from /proc/net/vlan/<interface> when available
• preserves existing support for numeric dotted names such as eth0.9
• handles names such as p6p3.vlan9
• returns no VLAN instead of raising an exception for unrelated dotted interface names
• falls back safely if kernel VLAN metadata is unavailable or unusable

Tests

Added regression coverage for:

• kernel-reported VLAN IDs such as p6p3.vlan9
• existing numeric suffixes such as eth0.9 and eno1.50
• non-numeric dotted interface names
• unusable kernel VLAN metadata

Validation completed successfully:

4 VLAN regression tests passed
7 tests/network.py tests passed
ruff check passed
ruff format --check passed