fix(librenms-health): report numeric sensors that breach their limits

[NavidSassan]

Problem

librenms-health derived each sensor's state solely from LibreNMS' state_generic_value, which LibreNMS populates only for discrete state-class sensors (fan ok/fail, psu present/absent, ...). For numeric sensors (temperature, humidity, voltage, power, ...) that column is NULL, so the plugin forced them to OK and every over- or under-limit reading was silently reported as OK. A humidity sensor reading 71.7 with a high limit of 70 showed [OK] while LibreNMS itself flagged it critical.

This is not a regression: numeric-limit alerting was never implemented (the pre-2024 API version never set a state at all).

Fix

Mirror LibreNMS' own Sensor::currentStatus() so the check agrees with the severity shown on the LibreNMS device health pages:

• CRITICAL at or beyond the hard sensor_limit / sensor_limit_low
• WARNING at or beyond the soft sensor_limit_warn / sensor_limit_low_warn
• UNKNOWN for a state sensor without a translation, or a numeric sensor without a reading
• comparisons are inclusive (>= / <=), matching LibreNMS

The decision lives in a pure get_sensor_state() helper, computed before the row values are turned into display strings.

Output

The value column now shows both limit ranges as low_warn..high_warn/low_crit..high_crit (header Val (warn/crit Range)). An unset range renders as - so the remaining one stays unambiguous — LibreNMS auto-discovery commonly configures only the critical limits, so many sensors legitimately show only a critical range:

Hostname     ! SysName  ! Sensor                   ! Val (warn/crit Range)        ! State
-------------+----------+--------------------------+------------------------------+-----------
192.0.2.11   ! pdu-a1   ! lgpEnvSupplyAirHumidity  ! 71.7 (35.0..65.0/30.0..70.0) ! [CRITICAL]
192.0.2.11   ! pdu-a1   ! lgpEnvReturnAirHumidity  ! 82.0 (-/20.0..70.0)          ! [CRITICAL]
storinator02 ! synoRZ04 ! Disk 17 MZILT1T9HAJQ/007 ! NotInitialized               ! [WARNING]

Tests

Adds a --test seam plus a 15-case unit-test suite (JSON row fixtures, no live database) covering the warn/crit low and high bands, inclusive boundaries, single-sided limits, the - placeholder, state sensors, missing readings, mixed batches and --always-ok.

Also

• Drops three dead HTTP-era DEFAULT_* constants and the unused lib.librenms import.
• README updated (states, value-column explanation, and a note that warn limits are often unset by default but can be set per sensor in LibreNMS on the device Health tab).