fix(fox): cap the plan at the inverter Max SoC

[theluckystrike]

Fixes #5395

This follows approach 1 from the issue. There's a new per-inverter battery_max_soc arg in percent, default 100, and Fox's automatic_config() binds it to number.<prefix>_fox_<sn>_setting_maxsoc. Other integrations with a device-side cap can bind the same arg.

The cap goes into the planner ceiling, not into soc_max. soc_max stays the capacity because soc_kw comes from soc_percent * soc_max and the reserve is a percent of it, so scaling it would understate the stored energy.

After fetch_inverter_data() publishes the totals, apply_battery_max_soc_ceiling() adds up soc_max * battery_max_soc / 100 over the inverters. If any of them is below 100 it sets best_soc_max to that ceiling, or to the smaller of the two when the user already set one. It runs there and not in fetch_config_options() because the config read happens before this cycle's inverters exist. With every inverter at 100, best_soc_max is left exactly as it was.

The tests cover the Fox binding in test_fox_api.py, and the ceiling for one and two inverters plus the default of 100 in test_find_battery_size.py. The full unit_test.py run passes in about 200 seconds and black is clean on the changed files. docs/apps-yaml.md documents the new arg next to battery_min_soc.

I didn't look into the related point about maxSoc: 100 in the SelfUse groups, so that's still open.

Michael