Sum the in-range values of a search tree

Question

Add up every node value that falls inside a closed interval in a binary search tree supplied as a level-order sequence.

```python
def solve(raw: str) -> str: ...
```

## Requirements

- Line 1 lists the tree in level order, comma separated, with `null` marking a missing child.
- Line 2 holds `low,high`, both endpoints included.
- Return the sum of all node values `v` with `low <= v <= high` as a decimal string.
- Return `"0"` when the first token is `null` or when no value lies in the range.
- The listing describes a valid binary search tree.
- `run_case(case: dict)` forwards `case["stdin"]` to `solve`.
- Do not append a trailing newline to the returned string; return the value exactly once.