[php-nextgen]: Fix imports for models with parent schema

[coffeemakr]

The models with parent schema are missing imports.

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Summary by cubic

Adds missing imports to php-nextgen models with a parent schema to prevent undefined class errors and improve IDE support.

• Bug Fixes
  • Updated model.mustache to add InvalidArgumentException, ReturnTypeWillChange, and ObjectSerializer when parentSchema is present.
  • Regenerated affected PHP samples (e.g., DataQuery, Cat, Dog) to include the imports.

^{Written for commit 140fd45. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 7 files

[wing328]

thanks for the pr

cc @jebentier (2017/07), @dkarlovi (2017/07), @mandrean (2017/08), @jfastnacht (2017/09), @ybelenko (2018/07), @renepardon (2018/12)