Fix method/constant names for underscore-prefixed fields

[dereuromark]

Summary

• Fixes generated method and constant names for underscore-prefixed fields like _joinData and _matchingData
• Adds stripLeadingUnderscore filter to generate proper camelCase names
• Adds documentation for CakePHP special fields

Before

// Field: _matchingData
$dto->get_matchingData();
$dto->with_matchingData($data);
UserDto::FIELD__MATCHING_DATA;  // double underscore

After

// Field: _matchingData
$dto->getMatchingData();
$dto->withMatchingData($data);
UserDto::FIELD_MATCHING_DATA;  // single underscore

This is needed for CakePHP's projectAs() feature which uses _joinData (BelongsToMany pivot data) and _matchingData (matching query results).

Related to #51 which added support for underscore-prefixed field names.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.69%. Comparing base (416d131) to head (0b05f6d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #54      +/-   ##
============================================
+ Coverage     83.66%   83.69%   +0.02%     
- Complexity     1185     1187       +2     
============================================
  Files            34       34              
  Lines          2908     2913       +5     
============================================
+ Hits           2433     2438       +5     
  Misses          475      475              

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