fix: avoid deprecated device_registry.devices mapping access

[jmdevita]

Fixes #91.

Problem

PR #90 fixed the deprecated async_get_device(identifiers=...) lookup in cleanup_old_device() by replacing it with an iteration over device_reg.devices.values(). That swapped one deprecation for another.

In HA 2026.9, DeviceRegistry.devices returns a _DeprecatedDeviceRegistryItemsView. Iterating it is the supported use, but any mapping-style access — .values(), .get(), subscription — calls report_usage() and breaks in HA Core 2027.9.0. Hence the startup warning users see on 2026.9.*:

Detected that custom integration 'parcelapp' uses `device_registry.devices` as a mapping
or calls its lookup methods, which is deprecated; iterate it to get the device entries,
or use `async_get`, `async_entries_for_config_entry` and similar helpers for lookups

Fix

Walk this integration's own config entries and use dr.async_entries_for_config_entry(). Verified against the 2026.9.1 source that this helper reads registry._devices directly, bypassing the deprecated view, so it emits no warning:

def async_entries_for_config_entry(registry, config_entry_id):
    """Return entries that match a config entry."""
    return registry._devices.get_devices_for_config_entry_id(  # noqa: SLF001
        config_entry_id
    )

Two notes on the implementation:

• Iterating every config entry of the domain, rather than just the one being set up, preserves fix: replace deprecated device_registry.async_get_device (removed in HA 2027.8.0) #90's behaviour. cleanup_old_device() runs once globally (guarded by the processed_cleanup flag), and the legacy device may belong to an entry that isn't the first to set up.
• The removed set guards against calling async_remove_device() twice on a device shared by two entries, which would raise KeyError. Defensive — the malformed device predates multi-hub support, so in practice it only ever belonged to one entry.

Testing

Adds a regression test for cleanup_old_device(), which had no coverage despite being rewritten twice. 272 tests pass.

Worth flagging: the test does not catch this specific deprecation. The pinned pytest-homeassistant-custom-component resolves to HA 2026.3.1, which predates the compatibility view entirely, so the test passes against the old code too. It pins the behavioural contract — bare (DOMAIN,) is removed, (DOMAIN, entry_id) is kept — not the API choice.

Also syncs manifest.json to 1.8.3, matching the existing tag. The release workflow patches this from the tag at publish time, so shipped artifacts were already correct; this is repo bookkeeping only.