fix(gateway): read car battery size per slot as a float

[mgazza]

Problem

When a gateway EV charger doesn't report SoC, the gateway estimates it from the session energy and car_charging_battery_size. It reads that arg with an int default and no index:

battery_size_kwh = self.get_arg("car_charging_battery_size", 100)

car_charging_battery_size is a per-car arg (entries: num_cars), so this fails two ways:

• A list such as [10.5] or [sensor.my_car_battery_size] fails int coercion. get_arg then logs Return bad int value [...] and sets had_errors on every telemetry update, so the warning sticks in the status. It falls back to 100 kWh, and the SoC estimate is off by the ratio of the real battery to 100.
• A fractional size such as 10.5 kWh is truncated to 10 by the int default.

The planner already reads this arg with 100.0, index=car_n (fetch.py), so the gateway was the only unindexed reader.

Fix

battery_size_kwh = self.get_arg("car_charging_battery_size", 100.0, index=0)

Gateway EV auto-config places the charger in car slot 0, so read that slot with a float default. A scalar value still resolves as before, because index 0 on a non-list is a no-op.

Tests

The existing TestEvTelemetry tests replace get_arg with a lambda, so they can't see how the value is resolved. The new test_ev_soc_battery_size_through_get_arg runs _inject_ev_entities against the real PredBat.get_arg for four forms of the arg:

• a per-car list of numbers;
• a per-car list of entity ids;
• a single entity id;
• a single fractional number.

Each must give 50% for 5.25 kWh delivered into a 10.5 kWh battery, without setting had_errors. It runs from run_gateway_tests when a PredBat is passed, and restores the args, entity and status it touches.

• ./run_all --test gateway: passes, including the new test.
• Against the previous line, the new test fails: expected SoC 50.0 from a 10.5 kWh battery, got 5.2.
• pre-commit (ruff, black, cspell) passes on both files.

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