HID: input: Introduce struct hid_battery and refactor battery code

HID: input: Introduce struct hid_battery and refactor battery code

Introduce struct hid_battery to encapsulate individual battery state,
preparing for future multi-battery support.

The new structure contains all battery-related fields previously stored
directly in hid_device (capacity, min, max, report_type, report_id,
charge_status, etc.). The hid_device->battery pointer type changes from
struct power_supply* to struct hid_battery*, and all battery functions
are refactored accordingly.

A hid_get_battery() helper is added for external drivers, with
hid-apple.c and hid-magicmouse.c updated to use the new API. The
hid-input-test.c KUnit tests are also updated for the new structure.

No functional changes for single-battery devices.

Signed-off-by: Lucas Zampieri <lcasmz54@gmail.com>
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>