Fix: power capacity does not need to be mandatory

[BelhsanHmida]

Description

• Let storage scheduling continue when a flex-model omits power-capacity.
• Infer the symmetric device-level power limit from the greatest of consumption-capacity and production-capacity when both directional capacities are configured.
• Keep the existing site-power-capacity fallback after the directional-capacity fallback.
• Clarified the scheduling metadata for power-capacity, consumption-capacity, and production-capacity.
• Added changelog entry.

Look & Feel

N/A. This is scheduler behavior and documentation only.

How to test

pytest -k "device_power_capacity_uses_greatest_directional_capacity_before_site_fallback"

Manual check:

• With no power-capacity, production_capacity = 300 kW, and consumption_capacity = 700 kW, _get_device_power_capacity returned a 96-step capacity series whose values are close to 0.7 MW.
• The method logged the expected missing power-capacity warning instead of raising the previous error.

Related Items

Closes SeitaBV/ems#210

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Sign-off

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on code under GPL or other license that is incompatible with FlexMeasures

[Flix6x]

Thank you, great work.