fix: Caesar cipher produces non-alphabetic output for negative shifts

[herley-shaori]

Fixes #7506

Problem

Caesar.encode/Caesar.decode produced corrupted, non-alphabetic output for any negative shift that is not an exact multiple of 26:

new Caesar().encode("A", -1); // returned "@" (ASCII 64) instead of "Z"

Root cause: normalizeShift did (char) (shift % 26). Java's % preserves the dividend's sign, so -1 % 26 == -1, and casting a negative int to the unsigned 16-bit char type wraps it to 65535. The following char arithmetic then overflowed and truncated into an arbitrary wrong character, which the > 'Z' / < 'A' wraparound checks could not catch. This also broke the decode(encode(x, s), s) == x round-trip for negative shifts.

Fix

• normalizeShift now returns an int normalized into 0..25 via ((shift % 26) + 26) % 26, so a negative shift is mapped to its equivalent positive rotation (e.g. -1 → 25).
• The shift arithmetic in encode/decode is performed in int and cast back to char only when appending, which also removes the lossy-conversions compound-assignment pattern.

Tests

Added three regression tests to CaesarTest:

• caesarEncryptWithNegativeShiftTest — encode("A", -1) must wrap to "Z" (upper and lower case).
• caesarDecryptWithNegativeShiftTest — decoding with a negative shift.
• caesarNegativeShiftRoundTripTest — encode→decode round-trip for shifts -1, -5, -25, -26, -27, -52.

Verified locally: mvn test -Dtest=CaesarTest → 6/6 pass, mvn checkstyle:check clean, clang-format --dry-run --Werror clean. All existing tests still pass unchanged.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.25%. Comparing base (c6fa50f) to head (5625af5).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7508      +/-   ##
============================================
+ Coverage     80.23%   80.25%   +0.01%     
- Complexity     7355     7358       +3     
============================================
  Files           810      810              
  Lines         23785    23787       +2     
  Branches       4678     4678              
============================================
+ Hits          19085    19091       +6     
  Misses         3940     3940              
+ Partials        760      756       -4     

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[DenizAltunkapan]

@herley-shaori thanks!