Add tree integrity validation to NestedSetsBehavior

[coderabbitai]

Overview

Add a comprehensive tree integrity validation method to detect structural issues in nested set operations.

Requirements

• Tree integrity validation method checking for gaps, overlaps, and boundary correctness
• Integration points after critical operations (deletion, cross-tree movement, root promotion)
• Comprehensive test coverage for edge cases and boundary conditions

Detailed Implementation

• Add validateTreeIntegrity() method in src/NestedSetsBehavior.php
• Validate no gaps or overlaps in lft/rgt values
• Check proper nesting relationships
• Verify depth consistency
• Integration after operations like afterDelete(), moveNode(), moveNodeAsRoot()

Test Coverage

• Adjacent siblings scenarios
• Single children edge cases
• First/last boundary nodes
• Cross-tree movement validation
• Root promotion integrity checks

Background

Suggested during review of PR #13 to enhance confidence in nested set operations and detect potential off-by-one errors.

Related PR: #13
Comment: #13 (comment)

[terabytesoftw]

done.