#24632 [Java] [okhttp-gson] Add `anyOf` Discriminator Support

Adds support for discriminators for anyOf models in the Java okhttp-gson library. This functionality is copied over from the oneOf model to match it, and is also gated behind the useOneOfDiscriminatorLookup flag. I kept it behind the flag since it seemed reasonable to assume users would want to toggle discriminator lookup for both oneOf+anyOf at once, tho the flag is now slightly inappropriately named. Lmk if we need to make a change regarding that, that flag is used across a few generators.

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Summary by cubic

Adds discriminator-based deserialization for anyOf models in the Java okhttp-gson generator, matching oneOf. It now performs fast discriminator lookup when present and safely skips it for non-object payloads, with clearer warnings for missing or non-primitive discriminator values.

• New Features

  • Added discriminator lookup in anyOf adapters with early switch and fallback to schema-order matching.
  • Guarded lookup to only run for object payloads; improved logs for missing/non-primitive discriminator values.
  • Updated petstore samples (MammalAnyof) and tests for mapped values, unmapped fallback, and empty payload failure.

• Migration

  • No migration required. Enable useOneOfDiscriminatorLookup to apply to both oneOf and anyOf.