feat(domains): add AirQuality domain (#34)

[Faerkeren]

Closes #34.

Issue validity

Valid feature request. Air quality entities are a distinct HA domain with multiple co-reported pollutant metrics that do not fit the single-value Sensor abstraction. Without a domain helper users must hand-parse attributes with no type safety or change listeners — inconsistent with the typed experience of Sensor, Climate, etc.

Fix

Adds `src/haclient/domains/air_quality.py`:

• `AirQuality` read-only `Entity` subclass registered as the `air_quality` domain.
• Typed metric properties: `air_quality_index`, `particulate_matter_2_5`, `particulate_matter_10`, `carbon_dioxide`, `carbon_monoxide`, `volatile_organic_compounds`, `nitrogen_dioxide`, `ozone`.
• Graceful degradation: every metric returns `None` when the underlying sensor doesn't report it. A shared numeric coercion helper accepts `int`/`float`/numeric strings and rejects bools, empty strings, `unknown`/`unavailable`, and unsupported types.
• `air_quality_index` prefers an explicit `air_quality_index` attribute when present, otherwise falls back to the entity state (which is what HA's `air_quality` platform sets.
• Listener decorators per the issue spec: on_aqi_change, on_pm25_change, on_co2_change.
• Registered & exported from haclient.domains.__init__, automatically reachable as ha.air_quality("name").

Tests added (tests/test_domains.py)

• test_air_quality_full_metrics — every metric reads its attribute.
• test_air_quality_degrades_when_metrics_missing — every metric returns None on an empty sensor.
• test_air_quality_index_prefers_explicit_attribute — attribute overrides state.
• test_air_quality_coercion_handles_strings_and_bad_values — numeric strings coerce, junk/bool/list yield None.
• test_air_quality_listeners — all three decorators fire correctly.

Validation

• ruff check src tests — clean
• ruff format --check src tests — clean
• mypy src (strict) — clean
• pytest tests/ --cov=haclient --cov-report=term-missing --cov-fail-under=95 — 265 passed, total coverage 96.62% (new module 100%).
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