Stop importing the deprecated CONCENTRATION_MICROGRAMS_PER_CUBIC_METER

HA logs a removal warning (2027.8) every time this name is accessed on
releases that carry UnitOfDensity, attributed straight to this
integration since it's a plain module-level import. UnitOfDensity is
the replacement, but hacs.json's floor (2025.1.0) predates it existing
at all -- pytest-homeassistant-custom-component 0.13.316, the newest
available, still has no UnitOfDensity either, so this can't be a
static import on either branch without breaking support for part of
the version range.

Resolved with a runtime getattr instead: reads UnitOfDensity off the
homeassistant.const module if present and uses its
MICROGRAMS_PER_CUBIC_METER member, otherwise falls back to the plain
(un-deprecated, on those older releases) constant. The getattr
short-circuits before the deprecated name is ever touched on a release
new enough to have UnitOfDensity, so the warning stops firing there
without dropping support for anything still on the old one. Same
feature-detection shape _relabel_particulate_statistics already uses a
few lines down for new_unit_class.

Verified the resolution logic directly: against the installed HA
(2026.2.3, pre-UnitOfDensity) it resolves to the plain constant with no
warning; a simulated future homeassistant.const with UnitOfDensity
present resolves to it without ever touching the deprecated name (a
guard that raises on that access never fires).

Full suite (1573 tests), ruff, and `ty check custom_components tests`
(CI's exact invocation) all pass.

custom_components/localthings/__init__.py

@@ -7,10 +7,8 @@ import logging
 import re
 from typing import Any
 
+from homeassistant import const as ha_const
 from homeassistant.config_entries import ConfigEntry
-from homeassistant.const import (
-    CONCENTRATION_MICROGRAMS_PER_CUBIC_METER as PARTICULATE_UNIT,
-)
 from homeassistant.const import EVENT_HOMEASSISTANT_STOP
 from homeassistant.core import Event, HomeAssistant, callback
 from homeassistant.exceptions import ConfigEntryNotReady
@@ -25,6 +23,22 @@ from .services import async_setup_services
 
 _LOGGER = logging.getLogger(__name__)
 
+# UnitOfDensity is the non-deprecated home for this value from whichever HA
+# release introduces it; CONCENTRATION_MICROGRAMS_PER_CUBIC_METER logs a
+# removal warning (2027.8) on those releases every time it's accessed.
+# hacs.json's floor (2025.1.0) predates UnitOfDensity existing at all, so
+# this is a runtime getattr rather than a static import ty could only ever
+# resolve against one HA generation -- see _relabel_particulate_statistics'
+# own new_unit_class feature-detection below for the same pattern. The
+# getattr short-circuits before the deprecated name is ever touched on a
+# release new enough to have UnitOfDensity.
+_unit_of_density = getattr(ha_const, "UnitOfDensity", None)
+PARTICULATE_UNIT = (
+    _unit_of_density.MICROGRAMS_PER_CUBIC_METER
+    if _unit_of_density is not None
+    else ha_const.CONCENTRATION_MICROGRAMS_PER_CUBIC_METER
+)
+
 
 async def async_setup(hass: HomeAssistant, config: ConfigType) -> bool:
     # Services are process-global, registered once here rather than per