Add unit tests for internal/difflib package

[Copilot]

Improves test coverage in internal/difflib from 85.0% to 92.5%.

New tests in difflib_test.go

• Caching: TestSequenceMatcherCaching, TestSetSeqSamePointer
• Junk filtering: TestSequenceMatcherWithIsJunk, TestJunkFilteringInChainB, TestFindLongestMatchWithJunk, TestMatchExtensionWithJunkOnBothSides, TestFindLongestMatchWithJunkExtension
• AutoJunk (n≥200): TestAutoJunkWithLargeSequence, TestAutoJunkPopularElements
• Edge cases: TestFindLongestMatchBreakCondition, TestFindLongestMatchEdgeCases, TestFindLongestMatchExtension, TestMatcherWithBothSequencesSame, TestWriteUnifiedDiffWithDefaultEol

New tests in options_test.go

• TestDefaultPrinterBuilder — covers previously untested export
• TestOptionsWithAllCustomPrinters — OtherPrinter/Formatter customization
• TestOptionsWithDefaults — nil and partial options handling

Functions now at 100% coverage

optionsWithDefaults, DefaultPrinterBuilder, defaultPrinterBuilder, defaultFormatterBuilder, GetMatchingBlocks, GetOpCodes, GetGroupedOpCodes, chainB, isBJunk

Remaining uncovered lines are error paths requiring mock io.Writer failures.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.00%. Comparing base (b33472c) to head (a82b98f).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
+ Coverage   96.63%   97.00%   +0.37%     
==========================================
  Files          53       53              
  Lines        8227     8227              
==========================================
+ Hits         7950     7981      +31     
+ Misses        184      162      -22     
+ Partials       93       84       -9     

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