fix: duplicated discriminator serialisation issue [Work In progress]

[YuriiBatkovych]

This PR addresses OpenAPI-Generator Issue #3796, where the discriminator property is serialized twice in Java Jackson projects.

The Problem

When using a discriminator with Jackson, the generator currently includes allowGetters = true within the @JsonIgnoreProperties annotation. This configuration inadvertently tells Jackson that even though the property is "ignored," it is still allowed to be accessed via its getter during the serialization process.

Because @JsonTypeInfo is also configured to include the discriminator (via visible = true and include = JsonTypeInfo.As.PROPERTY), Jackson ends up writing the field twice:

• Once from the explicit class property/getter.
• Once from the internal Jackson polymorphic handling.

The Fix

We have modified typeInfoAnnotation.mustache to remove allowGetters = true.

• Removal of allowGetters = true: This ensures that Jackson ignores the manual getter during serialization, preventing the duplicate field.

• Retention of allowSetters = true: This remains necessary to ensure that the discriminator can still be mapped to the class field during deserialization.