Harden generators and fix TS union imports

[dereuromark]

Summary

• add missing TypeScript imports for DTO references inside union types in multi-file output
• fail loudly when the TypeScript or JSON Schema generators cannot create output directories or write files
• add regression tests for both issues and note the generator error behavior in the CLI docs

Verification

• composer cs-fix
• vendor/bin/phpunit

[codecov]

Codecov Report

❌ Patch coverage is 72.85714% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.70%. Comparing base (fd6239d) to head (f6729f0).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Generator/TypeScriptGenerator.php	72.00%	14 Missing ⚠️
src/Generator/JsonSchemaGenerator.php	75.00%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #103      +/-   ##
============================================
+ Coverage     82.46%   82.70%   +0.24%     
- Complexity     1465     1486      +21     
============================================
  Files            42       42              
  Lines          3615     3660      +45     
============================================
+ Hits           2981     3027      +46     
+ Misses          634      633       -1     

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