[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

Fixes deserialization and typing for nested oneOf/anyOf in php-nextgen by flattening composed unions to their leaf types. Unifies handling via ComposedInterface and adds anyOf support to prevent TypeErrors with nested composed schemas.

• Bug Fixes

  • Flatten nested oneOf/anyOf unions so properties accept leaf member types during deserialization.
  • Add anyOf support to models and docs with a new model_anyof.mustache.
  • Unify composed handling with ComposedInterface; ObjectSerializer now dispatches via deserializeComposed before array-to-object casting.
  • Update tests and samples to cover nested and discriminator scenarios.

• Migration

  • OneOfInterface is replaced by ComposedInterface. Update any custom references and regenerate clients.

^{Written for commit 9d1ce0b. Summary will update on new commits.}