fix: add missing docstring on MathDelimiterProblem.__str__

pydocstyle (D105) was failing CI lint for the magic method missing a
docstring.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: m-messer

in2lambda/validation/delimiters.py

@@ -39,6 +39,7 @@ class MathDelimiterProblem:
     error: MathDelimiterError
 
     def __str__(self) -> str:
+        """Render as ``line <n>: <message>``."""
         return f"line {self.line}: {self.error.value}"