Use diff_match_patch for --fix dry-run output

[reidbaker]

This pr is the result of me experimenting with gastown. It still needs review from me.

Summary

• Replaces the naive index-based diff in _printDiff (validation_session.dart) with a line-tokenized diff backed by package:diff_match_patch.
• The previous implementation misrepresented any change that inserted or removed a line because it compared lines at the same index. The new computeLineDiff tokenizes each line to a unique code unit, runs diff_match_patch.diff on the token streams, and emits +/- entries with separate line numbers for the original and modified texts.
• Fixes [lint] Use package.diff for a better more aware differ for --fix #79.

Test plan

• 8 new unit tests in tool/dart_skills_lint/test/diff_printer_test.dart cover identical inputs, pure modification, pure insertion (end + middle), pure deletion, mixed modification + insertion, deletion followed by modification, and significant-whitespace preservation.
• All 118 tests in tool/dart_skills_lint (110 existing + 8 new) pass. The existing CLI integration test for --fix dry-runs and shows diff but does not modify file still passes through the new code path.
• dart analyze clean.

[gemini-code-assist]

Code Review

This pull request replaces the naive line-by-line diffing logic in ValidationSession with a more robust implementation using the diff_match_patch package. It introduces a new computeLineDiff utility that tokenizes lines to handle insertions and deletions correctly, accompanied by comprehensive unit tests. Feedback was provided to refactor computeLineDiff to eliminate redundancy in line storage and improve the clarity of the tokenization process.

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tool/dart_skills_lint/lib/src/validation_session.dart (553-601)

The current implementation of computeLineDiff has a few minor issues:

• Redundancy: lineArray is initialized with a dummy empty string at index 0, but lineToIndex is empty. This causes actual empty lines in the input to be stored twice in lineArray (at index 0 and a new index).
• Robustness: Mapping lines to UTF-16 code units using writeCharCode has a limit of 65,535 unique lines. Exceeding this will generate surrogate pairs, which diff_match_patch (working on UTF-16 strings) will treat as two separate characters, corrupting the diff. While unlikely for single files, it's a known limitation of this technique.
• Clarity: The dummy element at index 0 is used to avoid char code 0, but this is implicit. It's clearer to start with an empty array and explicitly offset the char code by 1.

The suggested refactor addresses the redundancy and improves clarity.

List<String> computeLineDiff(String original, String modified) {
  final lineArray = <String>[];
  final lineToIndex = <String, int>{};

  String tokenize(String text) {
    final buf = StringBuffer();
    for (final String line in text.split('\n')) {
      final int idx = lineToIndex.putIfAbsent(line, () {
        lineArray.add(line);
        return lineArray.length - 1;
      });
      // Use idx + 1 to avoid char code 0, which can be problematic for some diff engines.
      buf.writeCharCode(idx + 1);
    }
    return buf.toString();
  }

  final String tokenizedOriginal = tokenize(original);
  final String tokenizedModified = tokenize(modified);

  final List<dmp.Diff> diffs = dmp.diff(
    tokenizedOriginal,
    tokenizedModified,
    checklines: false,
  );

  final output = <String>[];
  var originalLineNumber = 1;
  var modifiedLineNumber = 1;

  for (final d in diffs) {
    final List<int> codes = d.text.codeUnits;
    switch (d.operation) {
      case dmp.DIFF_EQUAL:
        originalLineNumber += codes.length;
        modifiedLineNumber += codes.length;
      case dmp.DIFF_DELETE:
        for (final c in codes) {
          output.add('- Line $originalLineNumber: ${lineArray[c - 1]}');
          originalLineNumber++;
        }
      case dmp.DIFF_INSERT:
        for (final c in codes) {
          output.add('+ Line $modifiedLineNumber: ${lineArray[c - 1]}');
          modifiedLineNumber++;
        }
    }
  }
  return output;
}