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.}