Fix Generator path regex to tolerate either directory separator

[dereuromark]

Summary

Generator::findExistingDtos() and Generator::findExistingMappers() both matched on regex patterns with hardcoded forward slashes:

#src/Dto/(.+)<suffix>\.php$#
#Mapper/(.+)<suffix>Mapper\.php$#

RecursiveDirectoryIterator yields paths using the host OS separator, so on Windows where paths come back with backslashes the regex never matched — and the "delete generated DTOs no longer in the spec" pass silently no-op'd, leaving stale files behind on every regenerate.

Replacement pattern accepts either separator via a character class [/\\]. Captured nested paths are normalized to forward-slash form so the same nested entry (e.g. Sub/Foo on Linux, Sub\\Foo on Windows) doesn't land in the result map as two distinct keys depending on host OS.

How spotted

Surfaced while auditing dereuromark/cakephp-dto, which has its own (duplicated) Generator carrying the identical bug — already fixed in dereuromark/cakephp-dto#112. This PR ports the equivalent fix to the upstream library so other downstream users get it too.

Tests

Two new regression tests, one each for findExistingDtos and findExistingMappers. Exercising the Windows path directly on a Linux CI box is awkward, so each test drives the now-DS-tolerant regex via a temp-dir round trip on the host OS path layout (proves the common case still works) plus asserts that nested-directory entries get normalized to the canonical forward-slash form.

phpunit (809/809), phpstan, and phpcs all clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.07%. Comparing base (49ddfe2) to head (2481285).

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #112   +/-   ##
=========================================
  Coverage     83.06%   83.07%           
  Complexity     1555     1555           
=========================================
  Files            45       45           
  Lines          3833     3835    +2     
=========================================
+ Hits           3184     3186    +2     
  Misses          649      649           

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