Improve config merge behavior and add FieldKey constants

[dereuromark]

Summary

• Fixes configuration merge to properly merge fields at the field level instead of overwriting
• Adds FieldKey constants class to reduce magic strings throughout the codebase
• Sorts config files alphabetically in Finder for consistent merge order

Changes

Config Merge Fix

When the same DTO is defined in multiple configuration files, fields are now properly merged:

• Fields from all files are combined
• If the same field is defined in multiple files, later files' attributes override earlier ones
• Field attributes not specified in later files are preserved from earlier files

This enables modular configuration where you can:

config/dto/01-base-user.php      # Define base User DTO with all fields
config/dto/02-user-validation.php # Add validation rules to specific fields

FieldKey Constants

Introduces FieldKey class with constants for all field metadata keys:

• FieldKey::NAME, FieldKey::TYPE, FieldKey::REQUIRED, etc.
• FieldKey::metadataKeys() method for runtime metadata key list
• FieldKey::defaults() method for default field values

Updated Builder, DtoValidator, and FieldCompletor to use these constants.

Finder Sorting

Files in config/dto/ directory are now sorted alphabetically to ensure:

• Consistent merge order across different filesystems
• Ability to control merge order with numbered prefixes

Breaking Changes

None - this is a backwards-compatible enhancement.

[codecov]

Codecov Report

❌ Patch coverage is 94.59459% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.08%. Comparing base (6393236) to head (3ea5b24).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Generator/DtoValidator.php	81.25%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #96      +/-   ##
============================================
+ Coverage     82.63%   83.08%   +0.45%     
- Complexity     1409     1415       +6     
============================================
  Files            40       41       +1     
  Lines          3426     3476      +50     
============================================
+ Hits           2831     2888      +57     
+ Misses          595      588       -7     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.