Validate Rail Fence cipher rail counts

[krotname]

Motivation

• Prevent runtime crashes and undefined behavior when callers pass invalid rail counts to the Rail Fence cipher routines by failing fast with a clear error.
• Ensure both encode and decode respect the kata precondition that rails >= 2 and provide regression coverage for invalid inputs.

Description

• Added a shared validateRailCount(int) helper that throws IllegalArgumentException when the rail count is less than 2 and invoked it from both encode and decode in src/main/java/kyu3/RailFenceCipher.java.
• Preserved existing traversal and reconstruction logic while avoiding prior arithmetic/null/indexing faults for invalid n values.
• Added a parameterized test shouldRejectInvalidRailCounts and invalidRailCounts test data to src/test/java/kyu3/RailFenceCipherTest.java and imported assertThrows to verify negative, zero, and single-rail inputs are rejected.

Testing

• Ran the unit tests with mvn -q -Dtest=RailFenceCipherTest test and the new tests passed.
• Ran the full Maven verification with mvn -q verify and the build completed successfully including test and verification phases.
• Existing round-trip and example tests in RailFenceCipherTest continue to pass after the changes.

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Codex Task

[krotname]

Superseded by the validated combined merge in #115: #115