Longest run of distinct characters

Question

Given a string, measure the longest contiguous stretch in which no character repeats. Report only the length of that stretch.

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

## Requirements

- The input is a single string, and the trailing newline is not part of the payload.
- A stretch must be contiguous, and every character inside it must be distinct.
- Return the length as a decimal string, not the slice itself and not its starting position.
- Characters compare exactly, so an uppercase and a lowercase letter are different characters.
- A string whose characters all match answers `1`, and the longest stretch may sit anywhere in the payload.
- The payload can run to a few thousand characters, so keep the pass linear: move a shrinking left edge forward instead of rechecking each window from scratch.
- `run_case` passes `case["stdin"]` to `solve`.