fix(netlink): guard string.decode() against AttributeError in NLA array mode

[PhinehasNarh]

Problem

When an NLA attribute is declared as *string (an array of strings, e.g. NL80211_ATTR_SCAN_SSIDS), the base class nla_base.decode() sets self.value to a list of child cell objects rather than raw bytes. nlmsg_atoms.string.decode() then unconditionally called:

self.value = self.value.decode('utf-8')

on that list, raising:

AttributeError: 'list' object has no attribute 'decode'

This produced a WARNING traceback logged on every NL80211_CMD_TRIGGER_SCAN event when debug logging was enabled. The attribute value was still parsed correctly after the first failed call (because cell.decoded was already set to True by the base class), so the error was non-fatal but noisy.

Repro from #1330:

import logging
import pyroute2

logging.basicConfig(level=logging.DEBUG)
nl80211 = pyroute2.netlink.nl80211.NL80211()
nl80211.bind()
nl80211.add_membership("scan")
for event in nl80211.get():
    print(event)

Triggers:

WARNING:pyroute2.netlink:decoding NL80211_ATTR_SCAN_SSIDS
WARNING:pyroute2.netlink:AttributeError: 'list' object has no attribute 'decode'

Fix

One-line guard: only call .decode('utf-8') when self.value is actually bytes. When _nla_array=True, the value is already a list of child cells and should be left as-is.

if isinstance(self.value, bytes):
    try:
        self.value = self.value.decode('utf-8')
    except UnicodeDecodeError:
        pass

Tests

Three unit tests added to tests/test_unit/test_nlmsg/test_string_nla_array.py:

• test_string_nla_array_decode_no_attributeerror: verifies decode does not raise
• test_string_nla_array_value_is_list: verifies the decoded value is a list
• test_string_nla_plain_decode_still_returns_str: verifies plain (non-array) string decode is unchanged

Run with:

make test session=unit

The tests pass on Linux. They cannot run on Windows due to fcntl being imported transitively through pyroute2/__init__.py, which is consistent with the rest of the test_unit suite.

Closes #1330.

[svinota]

Thanks, checking!