[java][jackson] fix double serialization of discriminator

[dschodits-secunet]

This change fixes this issue #22923

The implementation now includes a conditional @JsonTypeInfo which doesn't change default behavior, but if disableDiscriminatorFieldIgnore is set to true the include parameter will be set to JsonyTypeInfo.As.EXISTING_PROPERTY.
This will prevent the discriminator from being serialized twice.

@bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10) @martin-mfg (2023/08)

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Summary by cubic

Prevents double serialization of the discriminator in Java (Jackson) models by making @JsonTypeInfo include conditional. Defaults to PROPERTY; when discriminator field ignore is disabled, uses EXISTING_PROPERTY to avoid duplicate output. Fixes #22923.

• Bug Fixes
  • Update typeInfoAnnotation.mustache to use JsonTypeInfo.As.PROPERTY by default.
  • When disableDiscriminatorJsonIgnoreProperties/disableDiscriminatorFieldIgnore is true, use JsonTypeInfo.As.EXISTING_PROPERTY so Jackson reuses the existing discriminator field instead of adding another.

^{Written for commit 57c7156. Summary will update on new commits.}

[cubic-dev-ai]

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