refactor: move accessor from service to device

[lackas]

Splits the device-identity concern (installation/gateway/device IDs) from the HTTP/cache concern (OAuth, request building, caching). Service instances no longer hold an accessor — it lives on Device and PyViCareDeviceConfig and is passed at each call site.

Picked up from @CFenner's #628 on current master. The 8-month drift, especially #764's PACKAGE_NOT_PAID_FOR / DeviceCommunicationError / InternalServerError exception handling added to ViCareCachedService, made a fresh implementation cleaner than a rebase. Same design intent and API shape as #628 — closes #628.

Motivation

This is prep work for a gateway-scoped service that can serve multiple devices from a single bulk fetch (/features/installations/{id}/gateways/{serial}/features?includeDevicesFeatures=true). Today's per-device fetch_all_features() is called N times per coordinator refresh; with a gateway service it could collapse to 1.

Measured on my installation: the bulk endpoint covers every isEnabled=true feature on every device (deactivated features are excluded by default, but PyViCare's existing @handleNotSupported / isEnabled checks already make missing and disabled features behaviourally equivalent). So a per-gateway architecture is functionally lossless for HA's use case — see also home-assistant/core#173776 where this came up.

What changes

• Device.__init__(accessor, service) — stores self.accessor, threads it through getProperty / setProperty
• PyViCareDeviceConfig.__init__(accessor, service, ...) — device_id derived from accessor.device_id; all as*() constructors pass self.accessor through
• ViCareService and ViCareCachedService drop the accessor instance attribute; getProperty / setProperty / fetch_all_features / buildGetPropertyUrl / reboot_gateway take accessor as a parameter
• get_available_burners(device) (free function in PyViCareHeatingDevice) takes a device instead of a service — its 4 callers updated accordingly
• 21 sites in PyViCareRoomControl and 1 in PyViCareHeatingDevice that called self.service.getProperty/setProperty directly now go through the Device delegation (self.getProperty/setProperty), which was already the established pattern in the base class
• ViCareCachedService keeps the full defensive try/except for PyViCareNotPaidForError, PyViCareDeviceCommunicationError, PyViCareInternalServerError introduced in fix: catch PyViCareNotPaidForError in auto-detection and diagnostics #764

What stays the same

Public API surface used by consumers (HA Core, custom integrations):

• device.getProperty(name) / device.setProperty(name, action, data) — unchanged signatures
• DeviceConfig.as*() accessors — unchanged signatures, same return types
• DeviceConfig.getConfig() still returns the accessor

Code that constructs ViCareService / ViCareCachedService / PyViCareDeviceConfig directly will need the new signature. The PyViCare orchestrator itself handles the wiring.

Verification

• All 729 tests pass (30 skipped, unchanged from master)
• mypy PyViCare/ clean (the 3 missing-stub warnings are pre-existing env-only issues that CI handles)
• ruff check PyViCare/ tests/ — all checks passed

Credit

Originally proposed by @CFenner in #628 — this PR carries the same design forward on current master.

closes #628

[lackas]

Rebased to drop the deprecation-checker commit — it's now a separate single-purpose PR #775 since it's orthogonal to this refactor. The Format step will stay red here until #775 lands, same as on master since #707.