#25273 [Java][Spring] useRecords: oneOf members as records

Third step towards #10490, after #25238. With useRecords, a oneOf member now becomes a record that implements its interface (sealed with useSealed) and gets the discriminator getter of each interface above it, nested oneOf included. A member stays a class for the reasons already logged, or when a discriminator property is missing or nullable. With useRecords off the output is unchanged.

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Summary by cubic

With useRecords, members of a oneOf interface are now generated as Java records that implement the (sealed) interface, including an accessor for each discriminator getter declared by the interfaces above them.

• The accessor is annotated with @JsonProperty so Jackson serializes the discriminator once under its JSON name, including for enum discriminators and nested oneOf interfaces.
• A member stays a class for the existing reasons (inheritance, additionalProperties, byte[] properties, property names a record can't have, builder clashes, request-parameter binding, component count) and when a discriminator property is missing or nullable under openApiNullable.
• If two interfaces in the chain declare the same getter name for different properties, the member stays a class as well.
• Added tests and regenerated samples; the spring-boot-records petstore sample now includes Animal and Zoo oneOf examples.