feat(core): Set variant in assistant messages too

[shantur]

What does this PR do?

Adds model variant selection in assistant messages

If you paste a large clearly AI generated description here your PR may be IGNORED or CLOSED!

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"
},