Serializable HTTP request representation.

Mimics Starlette/FastAPI Request but is pickle-able and contains only a safe
subset of data. Excludes session and auth to prevent exposing sensitive data.

Attributes:
    url: Serialized URL with path, port, scheme, netloc, query, hostname.
    base_url: Serialized base URL.
    headers: Request headers (marimo-specific headers excluded).
    query_params: Query parameters mapped to lists of values.
    path_params: Path parameters from the URL route.
    cookies: Request cookies.
    meta: User-defined storage for custom data.
    user: User info from authentication middleware (e.g., is_authenticated, username).