maths/largest_of_very_large_numbers: validate positive input

The docstring already documents that res(-1, 5) should raise
`ValueError: expected a positive input`, but no such validation
exists. Calling res() with a negative x currently reaches
`math.log10(x)` and raises `ValueError: math domain error` instead,
so the documented doctest fails.

Add the missing check at the top of the function so it matches
the documented contract.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: thejesh23

maths/largest_of_very_large_numbers.py

@@ -17,6 +17,8 @@ def res(x, y):
     ...
     ValueError: expected a positive input
     """
+    if x < 0:
+        raise ValueError("expected a positive input")
     if 0 not in (x, y):
         # We use the relation x^y = y*log10(x), where 10 is the base.
         return y * math.log10(x)