Update IsolationForest.php

[itashia]

refactor: improve type safety, readability, and modernize code

• Added precise type hints and docblocks:
  • Used @var array for float arrays
  • Defined return types with @return array<string, mixed>
• Extracted repetitive logic into private methods:
  • updateVariancesWithSmoothing()
  • calculateThreshold()
  • updateThreshold()
• Simplified model training checks via trained() method
• Minor optimizations:
  • Replaced or with || for consistency
  • Removed redundant and unnecessary comments
  • Used empty() for array checks
  • Combined mathematical operations for better performance
• Modernization changes:
  • Marked class as final to prevent unnecessary inheritance
  • Removed obvious comments (e.g. empty array declarations)
  • Sorted use statements for clarity