fix: ensure variants also work for completely custom models (#6481)

Co-authored-by: Daniel Smolsky <dannysmo@gmail.com>

packages/opencode/src/provider/models.ts

@@ -60,6 +60,7 @@ export namespace ModelsDev {
     options: z.record(z.string(), z.any()),
     headers: z.record(z.string(), z.string()).optional(),
     provider: z.object({ npm: z.string() }).optional(),
+    variants: z.record(z.string(), z.record(z.string(), z.any())).optional(),
   })
   export type Model = z.infer<typeof Model>