[php-nextgen] Fix Nested oneOf/anyOf Behavior

[ckoegel]

This PR builds off of work done in #24306. It flattens composed union type-hints transitively so a nested composed property resolves to its leaf members. Previously, composed property type-hints only listed their direct members, so when a composed type nested another composed type, the deserializer returned a leaf instance the property's union rejected and threw a TypeError.

Once #24306 is merged, I'll update this and mark as ready for review.

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Summary by cubic

Adds proper anyOf support and fixes nested oneOf/anyOf deserialization in php-nextgen by flattening unions to leaf types and dispatching via dedicated interfaces. Updates templates, docs, serializer, tests, and samples to handle discriminators and arrays correctly.

• Bug Fixes

  • Split composed handling into OneOfInterface and new AnyOfInterface; ObjectSerializer now calls deserializeOneOf/deserializeAnyOf before array-to-object casting.
  • Flatten nested oneOf/anyOf unions transitively so properties and docs use leaf member types, including discriminator cases.
  • Generate anyOf wrappers with model_anyof.mustache and AnyOfInterface.mustache; update model_doc.mustache and test templates to reflect composed behavior; add generator tests and sample models.

• Migration

  • No change to OneOfInterface; AnyOfInterface is new for anyOf wrappers. Regenerate clients.

^{Written for commit 1582a69. Summary will update on new commits.}