feat: add SensorLibrary entries for air-quality device classes

[bluetoothbot]

What

Add SensorLibrary convenience descriptions for the six air-quality device classes that were defined in SensorDeviceClass but had no library entry: CO, NITROGEN_DIOXIDE, NITROGEN_MONOXIDE, NITROUS_OXIDE, OZONE, SULPHUR_DIOXIDE.

Why

SensorLibrary already exposes FORMALDEHYDE and VOLATILE_ORGANIC_COMPOUNDS in the same concentration family, but the other standard Home Assistant air-quality pollutants were missing — so downstream BLE parsers had to hand-build a BaseSensorDescription instead of using the library. This fills that gap.

How

• Each new constant mirrors the existing pattern and uses its canonical HA unit: CO → ppm (matching CO2), the rest → µg/m³ (matching FORMALDEHYDE/VOC).
• No changes to SensorDeviceClass or Units — those values already existed; this is purely additive convenience.
• Inserted alphabetically near their neighbours; no existing entries touched.

Testing

New tests/test_library.py parametrizes the six entries and asserts both their metadata and the SensorUpdate produced through update_predefined_sensor. Full suite green (20 passed); ruff check and ruff format --check clean.

🤖 Generated with Claude Code

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Quality Report

Changes: 2 files changed, 123 insertions(+)

Code scan: clean

Tests: passed (20 passed)

Branch hygiene: clean

Generated by Kōan post-mission quality pipeline

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.44%. Comparing base (31bd826) to head (eecbd67).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #138      +/-   ##
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+ Coverage   94.37%   94.44%   +0.06%     
==========================================
  Files          12       12              
  Lines         480      486       +6     
  Branches        5        5              
==========================================
+ Hits          453      459       +6     
  Misses         26       26              
  Partials        1        1              

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