test(validate): pin the repeated entity pointer for per-car sensor args

[mgazza]

car_charging_battery_size and car_charging_limit are sensor-typed keys with
entries: num_cars, so a scalar fails validation as soon as runtime num_cars > 1
("Invalid type, expected list"). Anything publishing one of these as a pointer to a
single entity therefore has to emit a list, and rely on two behaviours that nothing
currently pins:

• the validator trims a list longer than num_cars rather than rejecting it, so a
  fixed-length list is safe at any car count; and
• each slot is read by index, so repeating the same entity id across the list
  resolves per car.

This adds a case to test_validate_config covering both. It seeds one sensor and
points every slot at it, then checks num_cars 0, 1, 2 and 8 all validate, that a
bare scalar still fails at 2, and that each slot resolves through get_arg(..., index=slot) as a float — the fractional value matters, since an int default
silently truncates a 10.5 kWh battery to 10.

No production code changes.

Follow-up to #5117, which fixed the gateway's unindexed read of the same key. The
test was written alongside that fix but landed after it merged, so it needs its own
PR.