Follow up to #23985
This PR adds support for anyOf models with and without discriminators. It copies the implementation from the oneOf model and updates some of the naming/logic to refer to Composed instead of oneOf or anyOf specifically.

PR checklist

• Read the contribution guidelines.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
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@jebentier (2017/07), @dkarlovi (2017/07), @mandrean (2017/08), @jfastnacht (2017/09), @ybelenko (2018/07), @renepardon (2018/12)

Summary by cubic

Adds anyOf support to the PHP Nextgen generator by introducing AnyOfInterface and generated dispatcher models, and updates ObjectSerializer to reliably dispatch oneOf/anyOf with safer JSON decoding. Samples, docs, and tests are regenerated, including new petstore anyOf models.

• New Features

  • Generate anyOf dispatcher models via model_anyof.mustache; add AnyOfInterface and register it in PhpNextgenClientCodegen.
  • Add deserializeAnyOf() and dispatch for both OneOfInterface and AnyOfInterface in ObjectSerializer; dispatch runs after json_decode and before array-to-object casting.
  • Update model docs to list anyOf members and discriminator selection; skip per-property test stubs for composed models; add petstore Reptile, Smoothie, Lizard, Snake; regenerate echo and petstore samples; add anyOf polymorphism tests.

• Bug Fixes

  • Preserve original strings when JSON decode fails (including the "null" literal); catch Throwable during deserialization; tighten composed-type validation.

^{Written for commit 9add223. Summary will update on new commits.}