Refactor SuffixArraySlow/Med, expand test coverage

[williamfiset]

Summary

• Refactor SuffixArraySlow: fix package order, add file-level header with Big-O, replace FQN java.util.Arrays with import, rename min_len to minLen, make suffixes private, add Javadoc to construct()
• Refactor SuffixArrayMed: fix package order, add file-level header with prefix doubling algorithm explanation, replace FQN with import, make SuffixRankTuple private, add Javadoc, remove large block of commented-out code in main(), remove useless null assignments, improve inline comments
• Fix Big-O notation in both files to use explicit brackets: O(n*log(n)) instead of O(n log n)
• Expand test coverage (6 → 13 tests): null input, single/two char edge cases, int[] constructor, getTextLength(), sorted suffix verification, randomized cross-validation (200 random strings across all 3 implementations)
• Clean up test file: remove unused fields, empty @BeforeEach, wildcard import

Test plan

• All 13 suffix array tests pass across SuffixArraySlow, SuffixArrayMed, and SuffixArrayFast

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