Longest Substring Without Repeated Characters

Question

Given a string, return the length of its longest contiguous substring containing no repeated character.

```python
def longest_unique_substring_length(text: str) -> int: ...
```

The empty string returns `0`. Treat characters as the language runtime's Unicode characters. Target `O(n)` time and `O(k)` space, where `k` is the number of distinct characters in the active input.

## Staged follow-ups

1. Return the substring or its half-open index range, not just its length.
2. Permit each character to appear at most `m` times.
3. Process a stream in chunks without retaining the entire prefix.