feat: add variants toggle (#6325)

Co-authored-by: Github Action <action@github.com>

packages/opencode/src/session/message-v2.ts

@@ -1,8 +1,6 @@
 import { BusEvent } from "@/bus/bus-event"
-import { Bus } from "@/bus"
 import z from "zod"
 import { NamedError } from "@opencode-ai/util/error"
-import { Message } from "./message"
 import { APICallError, convertToModelMessages, LoadAPIKeyError, type ModelMessage, type UIMessage } from "ai"
 import { Identifier } from "../id/id"
 import { LSP } from "../lsp"
@@ -308,6 +306,7 @@ export namespace MessageV2 {
     }),
     system: z.string().optional(),
     tools: z.record(z.string(), z.boolean()).optional(),
+    variant: z.string().optional(),
   }).meta({
     ref: "UserMessage",
   })