chore: merge Marstek Venus A and D templates

[andig]

The Venus A and D templates were identical apart from the capacity and maxchargepower defaults. Venus D is now a product of the Venus A template.

• marstek-venus-a covers marstek-venus-d, so existing configs keep working
• the per model values (A 2.12 kWh / 1200 W, D 2.56 kWh / 2200 W) move from the defaults into the param help, since a merged template can only carry one default
• both params are rendered, so both become required. yaml configs that relied on the removed defaults now fail with missing required ... instead of rendering an empty value into the battery mode register writes

🤖 Generated with Claude Code

/cc @webalexeu

[sourcery-ai]

Hey - I've left some high level feedback:

• Given the note that maxchargepower can now render as an empty value in battery mode register writes, consider marking it as required or providing a safe fallback to avoid silently misconfigured devices.
• With the merged template removing the capacity default, verify that any logic depending on a non-empty capacity parameter handles the absence appropriately, or consider enforcing it as a required parameter for both Venus A and D.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Given the note that `maxchargepower` can now render as an empty value in battery mode register writes, consider marking it as `required` or providing a safe fallback to avoid silently misconfigured devices.
- With the merged template removing the `capacity` default, verify that any logic depending on a non-empty `capacity` parameter handles the absence appropriately, or consider enforcing it as a required parameter for both Venus A and D.

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