[jaxrs-spec] Fix builder field type for JsonNullable properties

[Ignacio-Vidal]

With generateBuilders=true, a property carrying
x-is-jackson-optional-nullable produces a jaxrs-spec model that does not
compile:

error: incompatible types: String cannot be converted to JsonNullable<String>

The pojo field is rendered as JsonNullable<T>, but the generated builder
declared the property with its raw type, so the builder copy constructor's
this.x = b.x mismatched:

private JsonNullable<String> microchipId = JsonNullable.<String>undefined();  // pojo field
...
this.microchipId = b.microchipId;   // <-- does not compile
...
private String microchipId;         // builder field

Fix

Declare the builder field as JsonNullable<T>, defaulting to undefined() (or
of(default) for a non-container with a default), mirroring the pojo field. The
builder's setter still takes the raw type and wraps it, mirroring the pojo's own
fluent setter — so the builder API is unchanged for callers.

Why this was not caught

No existing sample combines generateBuilders with a JsonNullable property.
Regenerating the jaxrs samples produces zero diff.

Reproduced on a minimal 3.0 spec with no records, oneOf or inheritance involved —
only generateBuilders=true plus the extension is required.

Tests

New testBuilderFieldMatchesJsonNullableFieldType_issue24561, which fails on
master and passes with the fix. JavaJAXRSSpecServerCodegenTest 152 green.

[Ignacio-Vidal]

Opened upstream as OpenAPITools#24561.