Stream Dataset Handler

The `TypedDict` has `total=False` set intentionally to:

- Allow partial configs to be created and merged together via `merge_configs`
- Support config propagation from parent to child runnables via
    `var_child_runnable_config` (a `ContextVar` that automatically passes
    config down the call stack without explicit parameter passing), where
    configs are merged rather than replaced

!!! example

    ```python
    # Parent sets tags
    chain.invoke(input, config={"tags": ["parent"]})
    # Child automatically inherits and can add:
    # ensure_config({"tags": ["child"]}) -> {"tags": ["parent", "child"]}
    ```