feat(core): Set variant in assistant messages too

[shantur]

What does this PR do?

Adds model variant selection in assistant messages

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Manual Testing

{
        "info": {
            "id": "msg_c350253fd0019aBhGUrMTWePEU",
            "sessionID": "ses_3cafea7a4ffeUaTmwCrT2sZWuY",
            "role": "assistant",
            "time": {
                "created": 1770415870973,
                "completed": 1770415872871
            },
            "parentID": "msg_c350253cc0013P2nFF9JZp240k",
            "modelID": "gpt-5.3-codex",
            "providerID": "openai",
            "mode": "build",
            "agent": "build",
            "path": {
                "cwd": "/Users/shantur/Coding/tasmotizer",
                "root": "/Users/shantur/Coding/tasmotizer"
            },
            "cost": 0,
            "tokens": {
                "input": 10988,
                "output": 33,
                "reasoning": 15,
                "cache": {
                    "read": 0,
                    "write": 0
                }
            },
            "variant": "high",
            "finish": "stop"
        },