add THERMOSTAT_FAN_ONLY_HVAC

[vmvarga]

Add THERMOSTAT_FAN_ONLY_HVAC exposed-feature quirk id

Summary

Adds a new exposed-feature id, THERMOSTAT_FAN_ONLY_HVAC, to zhaquirks/quirk_ids.py.
It is a hint a quirk can place in device.exposes_features to tell ZHA that the
thermostat actually implements SystemMode.Fan_only (0x07), so the climate entity
may expose HVACMode.FAN_ONLY.

# Hint that the thermostat actually implements SystemMode.Fan_only, so the
# climate entity may expose HVACMode.FAN_ONLY
THERMOSTAT_FAN_ONLY_HVAC = "thermostat_fan_only_hvac"

It lives in the existing # Exposed features section, next to SE_POLL_SUMMATION
and SIREN_BASIC, and follows the same convention.

Why

ZHA's Thermostat climate entity derives its hvac_modes from
controlSequenceOfOperation, which per the ZCL spec only covers heating/cooling
combinations and never includes FAN_ONLY. Enabling FAN_ONLY purely because a
device advertises the Fan Control cluster (0x0202) is unreliable: many devices
expose the cluster but silently ignore SystemMode.Fan_only (e.g. some Tuya HVAC
thermostats).

The companion ZHA change therefore gates HVACMode.FAN_ONLY behind an explicit
opt-in: the climate entity only exposes it when this feature id is present in
device.exposes_features. Per maintainer feedback on that PR, the feature id itself
belongs here in zha-quirks (where all other exposed-feature ids live and are
imported from), not in the zha repo.

Companion PR

• ZHA: zigpy/zha — switches the climate entity to
  from zhaquirks.quirk_ids import THERMOSTAT_FAN_ONLY_HVAC and bumps the
  zha-quirks pin to the release containing this change. That PR depends on this
  one being merged and released first.

Usage

A device quirk declares support by exposing the feature, e.g. with the v2 quirk
builder:

from zhaquirks.quirk_ids import THERMOSTAT_FAN_ONLY_HVAC

(
    QuirkBuilder(...)
    ...
    .exposes_feature(THERMOSTAT_FAN_ONLY_HVAC)
    .add_to_registry()
)

A follow-up PR will add this opt-in to known-good fan-capable thermostats.

Test plan

• THERMOSTAT_FAN_ONLY_HVAC is importable: from zhaquirks.quirk_ids import THERMOSTAT_FAN_ONLY_HVAC
• Value is "thermostat_fan_only_hvac" (matches the string ZHA previously used,
  so no behavioral change for any already-shipped quirk metadata)
• Lint/format passes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.07%. Comparing base (f3e619e) to head (b1041a6).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #5093   +/-   ##
=======================================
  Coverage   93.07%   93.07%           
=======================================
  Files         401      401           
  Lines       13306    13307    +1     
=======================================
+ Hits        12385    12386    +1     
  Misses        921      921           

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