Validate Roman numeral parser input

[krotname]

Motivation

• The fromRoman implementation unconditionally used new StringBuilder(romanNumeral) and unboxed MAP_FROM.get(c), which caused NullPointerExceptions for null or unsupported input, and it accumulated into an int allowing silent overflow for very large inputs.

Description

• Added a null check to fromRoman that throws IllegalArgumentException for null input.
• Added validation for unsupported characters by checking MAP_FROM.get(c) and throwing IllegalArgumentException when a character is not recognized.
• Changed the accumulator from int to long and returned Math.toIntExact(result) so values outside the int range trigger an ArithmeticException instead of wrapping.
• Added regression tests in src/test/java/kyu4/RomanNumeralsTest.java that assert proper exceptions for null, invalid characters, malformed input, and the integer-overflow boundary while keeping existing round-trip and conversion tests.

Testing

• Ran mvn -q -Dtest=kyu4.RomanNumeralsTest test and the targeted test class passed.
• Ran mvn test and the full test suite completed successfully with 601 tests passing and no failures.

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[krotname]

Закрываю как superseded by #116: изменения перенесены в одну проверенную ветку, все review findings учтены; локальный mvn verify прошёл полностью.