Allow disabling the seeder unit of cultivators with a seeder configuration (#989)

[helgehelge123]

Problem

Fixes #989.

The Horsch Finer 6 SL, Väderstad TopDown 600 and Dalbo Powerchain 800 gained a seeder unit configuration (vehicle type cultivatingSowingMachine) with game patch 1.8. Their sowing machine spec has needsActivation="true", so getAIRequiresTurnOn() returns true and Courseplay hides the optional sowing setting for them. As a result the seeder unit is always turned on by the base game AI and cannot be disabled, even when the user only wants to cultivate.

Changes

1. CpVehicleSettings: the optional sowing setting (optionalSowingMachineEnabled) is now also visible for implements that have both the SowingMachine and the Cultivator specialization, not only for passive sowing machines that don't require turning on. isOptionalSowingMachineSettingDisabled() mirrors the visibility check.

2. SowingMachineController: while a CP job is running and sowing is disabled by the user, an override of TurnOnVehicle.getAIRequiresTurnOn() returns false for these implements. Without this, TurnOnVehicle:getCanAIImplementContinueWork() fails for machines that require turning on but are turned off, so the driver lowers the implement and then stands still forever, waiting in WAITING_FOR_LOWER.

   ⚠️ The override intentionally checks the specializations directly instead of using the setting's getIsDisabled(): that would call isOptionalSowingMachineSettingVisible(), which itself calls getAIRequiresTurnOn(), resulting in an infinite recursion (stack overflow every update loop).

3. SowingMachineController:needsRefilling(): no seeds are requested when sowing is disabled, or when the seed fill unit has capacity 0. The Finer 6 SL seeder unit has no real seed tank (capacity="0", which per SowingMachine:getSowingMachineCanConsume() means it can always consume) — CP treated it as permanently empty and kept waiting for a refill that can never happen ("tank is empty" stop when sowing was enabled).

Behavior

• Setting "Sowing option" appears in the vehicle settings (seeder section) for cultivators with a seeder unit configuration.
• Default is off (consistent with the existing optional sowing semantics), so the driver cultivates only; enabling it makes the driver seed as before.
• Normal seed drills are unaffected: without the Cultivator specialization the setting stays hidden/disabled and getAIRequiresTurnOn() falls through to the original implementation.

Testing

Tested in SP on game version 1.10 with the Horsch Finer 6 SL (seeder unit configuration):

• Sowing option off: driver drives the course and only cultivates, seeder stays off, no refill request.
• Sowing option on: driver seeds, no more bogus "tank is empty" stop (capacity-0 tank).
• Manual driving and giants helper unaffected (override only active while a CP job is running).