add optional headers field to model config - #3546

{
  "provider": {
    "anthropic": {
      "api": "https://api.anthropic.com",
      "models": {
        "claude-sonnet-4": {
          "id": "claude-sonnet-4",
          "headers": {
            "anthropic-beta": "my-experimental-feature",
            "x-trace-id": "session-42"
          }
        }
      }
    },
    "opencode": {
      "models": {
        "claude-sonnet-4": {
          "headers": {
            "x-custom-routing": "gold-tier"
          }
        }
      }
    }
  }
}