maths/largest_of_very_large_numbers: validate positive input as docstring documents

[thejesh23]

Describe your change:

Fixes #14930

The docstring of res() in maths/largest_of_very_large_numbers.py already documents:

```

res(-1, 5)
Traceback (most recent call last):
...
ValueError: expected a positive input
```

but the corresponding check was never actually written. Calling res(-1, 5) reaches math.log10(-1) and raises ValueError: math domain error instead of the documented message, so the doctest fails under python -m doctest maths/largest_of_very_large_numbers.py on current master.

This PR adds the missing two-line guard so the code matches its documented contract:

```python
if x < 0:
raise ValueError("expected a positive input")
```

After the change, all four doctests in the file pass. The doctest itself was not changed — it already described the intended behaviour.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".