#25641 [core][java] allOf models keep the additionalProperties of their members

An allOf model dropped the additionalProperties of its members: petstore's Cat has an Address member with integer values and got no map. When the model does not state the keyword itself, its members now decide: the value schema they agree on, otherwise any type. A child whose parent has the map inherits it, as okhttp-gson's x-inherits-additional-properties does. Only generators with supportsAdditionalPropertiesWithComposedSchema change.

@bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608 @martin-mfg @KannaKim @AndreyVMarkelov

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Summary by cubic

Fixes allOf models dropping the additionalProperties their members declare, so petstore's Cat now carries a map with the value type its Address member specifies.

• The composed schema's own additionalProperties keyword wins over member declarations.
• When members disagree, the value schema falls back to any type.
• A member stating additionalProperties: false vetoes the map, since allOf is an intersection.
• Children inherit a parent's map instead of redeclaring their own.
• Only generators with supportsAdditionalPropertiesWithComposedSchema change.

Builders

• Java and Spring builder copy paths (toBuilder()/copyOf()) now carry the additionalProperties map, including maps inherited from a parent.