Add test coverage for ReportCommand and Report classes

[Copilot]

PR #10 introduced ReportCommand and Report classes without corresponding test coverage. This PR adds comprehensive unit tests matching the repository's established testing patterns.

Changes

Report class tests (9 tests)

• Classification logic for all supported types (interfaces, exceptions, entities, value objects, uncategorized)
• Immutability and state accumulation behavior

ReportCommand tests (10 tests)

• Command execution with various input scenarios (valid folder, custom output dir, non-existent paths)
• File generation validation (index.html, report.js, chart.js, styles.css)
• JSON/HTML structure and content verification
• Error handling and graceful failure
• Directory creation for nested paths

Test infrastructure

• ReportNormalizer for serializing readonly Report instances in tests
• Temporary directory management with proper cleanup
• Type-safe file operations (handles file_get_contents() and getcwd() returning false)

Test results

• 35 total tests (16 original + 19 new), 140 assertions
• PHPStan and PHP-CS-Fixer passing

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