Refactor MagicEntryParser and address several PMD issues

[ekramHoque]

Summary

This pull request refactors parts of the project to improve readability, maintainability, and overall code quality while preserving existing behavior.

Refactorings

• Refactored MagicEntryParser.parseLine() by extracting helper methods with single responsibilities.
• Replaced the endian type switch logic with a lookup map to simplify conditional logic and improve extensibility.
• Encapsulated ContentData fields in MagicEntry using private fields and accessor methods.

Bug Fixes

• Replaced incorrect String identity (==) comparisons with .equals().
• Added null checking for File.listFiles() to prevent potential NullPointerException.
• Improved thread-safe initialization of internalMagicEntries using volatile and double-checked locking.
• Forwarded IOException to ErrorCallBack when reading directory magic files.
• Reported unknown special extension keys through ErrorCallBack instead of silently ignoring them.

Code Quality Improvements

• Addressed several PMD-reported issues including:

  • Collapsed nested if statements.
  • Removed redundant modifiers in interfaces and enum constructors.
  • Improved empty catch blocks with explanatory comments where intentional.
  • Applied additional small readability and maintainability improvements.

Validation

• Successfully built the project with Maven.
• Executed the full test suite successfully.

Tests run: 361
Failures: 0
Errors: 0
Skipped: 2

No functional behavior was intentionally changed; the modifications focus on refactoring and code quality improvements.