fix(wifi): honour a partially configured set of signal thresholds - #3691

The defect

WifiPlugin.get_alert overrides the base implementation and chains its three comparisons (glances/plugins/wifi/__init__.py:146):

try:
    if value <= self.get_limit('critical', stat_name=self.plugin_name):
        ret = 'CRITICAL'
    elif value <= self.get_limit('warning', stat_name=self.plugin_name):
        ret = 'WARNING'
    elif value <= self.get_limit('careful', stat_name=self.plugin_name):
        ret = 'CAREFUL'
except (TypeError, KeyError):
    # Catch TypeError for issue1373
    ret = 'DEFAULT'

get_limit returns None for a level the config does not define (plugin/model.py:969). value <= None raises TypeError, the blanket handler catches it, and the whole result collapses to DEFAULT.

Because critical is tested first, omitting it silences the other two as well.

Measured on develop

all three thresholds configured:
  quality_level= -50 dBm -> OK
  quality_level= -70 dBm -> CAREFUL
  quality_level= -80 dBm -> WARNING
  quality_level= -90 dBm -> CRITICAL

only 'critical' configured (careful/warning left out):
  quality_level= -50 dBm -> DEFAULT      <- a strong signal is not even OK
  quality_level= -90 dBm -> CRITICAL

only 'careful' configured:
  quality_level= -50 dBm -> DEFAULT
  quality_level= -70 dBm -> DEFAULT      <- past the careful threshold, no alert

For comparison, the base class handles partial thresholds:
  sensors base get_alert(95) with only 'critical' -> CRITICAL

A user who sets just wifi_careful in glances.conf gets no wifi alert at all, silently.

The fix

Each level is read once and guarded on its own, the way GlancesPluginModel.get_alert already does (plugin/model.py:854-862) — and the way the sensors plugin is already tested for (test_partial_type_thresholds_are_used, test_partial_sensor_thresholds_are_used).

Two details worth calling out:

• is not None, not truthiness. The base class uses if critical and value >= critical. These limits are negative dBm values, so a 0 threshold would be discarded by a truthiness test; is not None is exact.
• The TypeError handler is kept but narrowed. It was added for issue TypeError on opening (Wifi plugin) #1373 (a non-numeric signal level). With the None comparisons gone that is now the only way it can fire, so it still does its original job and nothing else. KeyError is dropped because get_limit returns None rather than raising.

After:

only 'careful' configured:
  quality_level= -50 dBm -> OK
  quality_level= -70 dBm -> CAREFUL

Tests

New tests/test_plugin_wifi.py, 13 cases:

• all four levels with a full config;
• each level alone still alerts (careful only, warning only, critical only);
• a strong signal is OK under any partial config, not undecorated;
• the most severe matching level wins;
• no thresholds at all stays DEFAULT;
• a non-numeric level stays DEFAULT — the TypeError on opening (Wifi plugin) #1373 guard.

5 of the 13 fail on develop and all 13 pass with the patch.

$ pytest tests/test_plugin_wifi.py -q
13 passed