Prevent SquareDigit overflow crash

[krotname]

Motivation

• The squareDigits method concatenated squared digits and called Integer.parseInt without checking range, causing NumberFormatException for inputs like 99999 when the constructed decimal exceeds Integer.MAX_VALUE.
• The change aims to make the method robust for arbitrary int inputs while preserving existing behavior for values that fit in a Java int.

Description

• Add a MAX_INT string constant and a pre-parse guard that compares the generated decimal string to Integer.MAX_VALUE by length and lexicographic order in src/main/java/kyu7/SquareDigit.java.
• If the generated result would exceed the int range the method now returns Integer.MAX_VALUE to avoid NumberFormatException while keeping representable results unchanged.
• Replace the previous direct parse with the guarded Integer.parseInt call on the validated string and remove reliance on unchecked parsing behavior.
• Add two unit tests in src/test/java/kyu7/SquareDigitTest.java that verify a normal case (9119) and the overflow/cap behavior for the reported 99999 input.

Testing

• Ran mvn -B -Dtest=SquareDigitTest test and the SquareDigitTest suite passed (3 tests, 0 failures).
• Ran the full verification mvn -B verify and the build succeeded, including JaCoCo coverage checks, Checkstyle, PMD, and SpotBugs.
• The added unit tests exercise both the representable-path and the overflow-saturation-path and succeeded under the CI-quality gate.

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Codex Task

[krotname]

Закрываю как superseded by #116: изменения перенесены в одну проверенную ветку, все review findings учтены; локальный mvn verify прошёл полностью.