تولید تست واحد از یک تابع موجود

Generate Vitest unit tests for the function below.
Coverage rules:

- One test per branch in the function.
- One edge case per input parameter (empty, max, negative,
  unicode, etc. — pick what applies).
- Two failure-mode tests (invalid input, dependency error).
- Use real values, not "foo"/"bar". Names should describe behavior.

Return one .test.ts file. No explanation outside the code block.

Function:
```ts
[paste function]
```