Add comprehensive test suite and improve code maintainability 🧪

[Copilot]

This PR significantly enhances the robustness of the git-diff-action by addressing the testing gaps and maintainability issues outlined in the original issue.

Changes Made

🐛 Bug Fixes

• Fixed undefined variable bug in git-diff.js where maxBufferSize was referenced before definition
• This bug would have caused issues in debug logging for buffer size validation

📈 Test Coverage Improvements

• Increased test suite from 9 to 33 comprehensive tests
• Achieved 100% branch coverage (improved from 89.47%)
• Added comprehensive edge case testing for all input options:
  • search_path: directory paths, glob patterns, specific files
  • git_options: custom flags, minimal options, complex combinations
  • git_diff_file: empty diffs, special characters, renames, mode changes
  • max_buffer_size: invalid values, boundary conditions, very large sizes
  • base_branch: custom branches, SHA commits
  • file_output_only: true/false scenarios with various output combinations

🧪 New Test Categories

• Error handling: File system errors, git command failures, malformed content
• Real-world scenarios: Complex configurations with multiple options
• Edge cases: Large diffs, Unicode characters, binary files
• Boundary testing: Empty inputs, maximum values, invalid parameters

🛠️ Maintainability Improvements

• Extracted magic numbers and strings into constants (DEFAULT_MAX_BUFFER_SIZE, GIT_DIFF_MARKER)
• Added helper function getMaxBufferSize() for input validation
• Improved variable naming (gitDiff → rawGitDiff to avoid confusion)
• Enhanced code comments and documentation
• Removed duplicate debug statements

📁 New Test Fixtures

• empty.diff: For testing empty diff scenarios
• special-chars.diff: Unicode and special character handling
• rename-and-mode.diff: File renames and permission changes
• large.diff: Performance testing with larger diffs

Test Results

Test Suites: 3 passed, 3 total
Tests:       33 passed, 33 total
Coverage:    100% statements, 100% branches, 100% functions, 100% lines

Example of New Test Coverage

// Edge case: Custom search path with glob pattern
test('executes gitDiff with glob pattern search_path', async () => {
  process.env.INPUT_SEARCH_PATH = '**/*.js'
  // ... validates git command is built correctly
})

// Error handling: File system errors
test('handles file system errors when writing output files', async () => {
  process.env.INPUT_RAW_DIFF_FILE_OUTPUT = '/invalid/path/raw.txt'
  // ... validates error is handled gracefully
})

These changes provide much greater confidence in the action's reliability across different usage scenarios and edge cases, while making the codebase more maintainable for future development.

Fixes #41.

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