Kostal Plenticore: add holdcharge implementation

[iseeberg79]

This implementation adds the holdCharge feature to Kostal Plenticore devices, as introduced in #27906.
It utilizes register 1038 to set a charge power limit (in watts), effectively preventing charging while still allowing discharge if required.

From the Kostal Modbus documentation:

Register	Reg. dec.	Description	Unit	Type	Lenth	Access	Mode
0x40E	1038	Battery max. charge power limit, absolute	W	Float	2	RW	0x03/0x10
0x410	1040	Battery max. discharge power limit, absolute	W	Float	2	RW	0x03/0x10

The existing defer logic is used to ensure registers written are properly reset when switching modes.

[iseeberg79]

@deadrabbit87 fyi

[sourcery-ai]

Hey - I've found 1 issue

Prompt for AI Agents

Please address the comments from this code review:

## Individual Comments

### Comment 1
<location path="templates/definition/meter/kostal-plenticore-gen2.yaml" line_range="135" />
<code_context>
           error: ErrNotAvailable
           {{- end }}
+      - case: 4 # holdcharge
+        set:
+          source: const
+          value: 0 # W
</code_context>
<issue_to_address>
**issue (bug_risk):** The nested `set:` key will override the outer `set` mapping, likely breaking the intended behavior.

Using multiple `set:` keys or nesting them incorrectly can cause earlier definitions (like `source: const / value: 0`) to be ignored by YAML/template processors, so the `holdcharge` behavior may not run as intended. Please ensure the modbus write (if needed) is either a separate list item (another `- set:`) or expressed under a different key at the same level, so both operations are handled as expected.
</issue_to_address>

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