fix(cover): declare _attr_is_closed on HotataRailCover

[RainySat]

Problem

HotataRailCover never defines _attr_is_closed, but HA's CoverEntity declares it with a type annotation and no default — unlike its neighbours in the same block (components/cover/__init__.py, HA 2026.9.2):

217:    _attr_current_cover_position: int | None = None
220:    _attr_is_closed: bool | None          # annotation only, no default
221:    _attr_is_closing: bool | None = None
222:    _attr_is_opening: bool | None = None

and is_closed is a cached_property that reads it directly:

@cached_property
def is_closed(self) -> bool | None:
    return self._attr_is_closed

So on a P5 airer every state write failed:

ERROR (MainThread) [custom_components.hotata.coordinator] Unexpected error updating listener 4340703627984 for hotata
Traceback (most recent call last):
  ...
AttributeError: 'HotataRailCover' object has no attribute '_attr_is_closed'

Two details make it noisy rather than intermittent:

• is_closed is reached twice per write — state() (if (closed := self.is_closed) is None:) and state_attributes() (data[IS_CLOSED] = self.is_closed)
• cached_property does not cache a raised exception, so it re-raises every time → one error per poll (measured: every 30 s, 54 lines in two minutes)

Only HotataRailCover is affected; HotataAirerCover and _HotataCurtainBase both implement is_closed.

改了啥

class HotataRailCover(HotataEntity, CoverEntity):
    _attr_device_class = CoverDeviceClass.SHADE
    # These rails have no position feedback, so is_closed can only be unknown.
    # CoverEntity annotates _attr_is_closed without a default (unlike
    # _attr_is_closing / _attr_is_opening), so omitting it here makes the base
    # is_closed cached_property raise AttributeError on every state write.
    _attr_is_closed: bool | None = None

None is the correct value here, not a convenient fallback. The A/B poles only report APoleMotorControlMode / BPoleMotorControlMode (open / stop / close) and have no position feedback, so "closed" is genuinely unknown — the same reason HotataAirerCover.assumed_state reports True when Position is not a real enum. CoverEntity.is_closed is typed bool | None and state() has an explicit None branch, so HA renders the entity as unknown, which is what it renders today. Guessing True/False would feed HomeKit and automations a wrong state.

怎么测

• py_compile passes for every module under custom_components/hotata/
• Deployed against a live HA 2026.9.2 instance running integration v4.0.3 (P5 airer with A/B poles): restarted, then watched 3+ poll cycles → 0 Unexpected error updating listener lines (previously ~1 per 30 s)
• 9.5 h later: still 0. The only hotata line in between is an unrelated api.link.aliyun.com … Network unreachable during a WAN outage, which recovered on its own
• A/B pole entities still report unknown (no position feedback — expected); open / close / stop are unaffected
• Swept the whole integration for the same class of bug (HA base classes annotate 53 _attr_* without defaults across 12 entity types) — this was the only missing one

关联 Issue

N/A — found on my own instance; searching is_closed in this repo returns 0 results, so I don't think it has been reported.

Checklist

• py_compile 全部通过
• hassfest CI 绿 — will rely on your CI
• HACS CI 绿 — same
• manifest.json 版本号已同步 — left to you, I didn't want to pick a version number
• CHANGELOG 已更新 — left to you; happy to add an entry (or an [Unreleased] section) if you'd rather have it inside the PR