[php][php-nextgen] Mark nullable things as nullable in phpdoc

[JulianVennen]

This is kind of a follow-up to #22310. There was a bunch of cases where only required was checked when determining nullability in PHPDoc. I have converted these to notRequiredOrIsNullable.
php-nextgen also now uses exts instead of vendorExtensions.

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Summary by cubic

Correctly mark nullable params and properties as nullable in generated PHPDoc for PHP and php-nextgen clients, improving type clarity. Also switch php-nextgen templates from vendorExtensions to exts.

• Bug Fixes

  • Use notRequiredOrIsNullable to add |null in PHPDoc for params and model properties.
  • Update samples (e.g., SplFileObject|null for binary body).

• Refactors

  • Replace vendorExtensions.* with exts.* across php-nextgen templates (api, docs, tests, models).
  • Align return/param types and test operation IDs to exts keys.

^{Written for commit ab7b185. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 8 files