[java][microprofile] Don't emit JsonNullable helper calls that are never defined

[Ignacio-Vidal]

The microprofile client does not support the Jackson Nullable library — the
generator documents this itself:

openApiNullable — Enable OpenAPI Jackson Nullable library. Not supported by microprofile library.

So a property carrying x-is-jackson-optional-nullable is still rendered as a
plain field. But equals/hashCode emitted equalsNullable(...) /
hashCodeNullable(...) for it anyway, while the helper definitions are gated
on a different, model-level extension (x-jackson-optional-nullable-helpers)
that core only sets from the schema's own nullability. When the two disagree the
generated model does not compile:

error: cannot find symbol
  symbol:   method equalsNullable(String,String)
  location: class CatResponseV2

When this happens

The two extensions diverge whenever a spec author writes
x-is-jackson-optional-nullable by hand — which is the only way to request a
JsonNullable property from the jaxrs-spec generator, since
AbstractJavaCodegen.isAddNullableImports() is called solely by
JavaClientCodegen and SpringCodegen.

Vendor extensions live in the spec, so they apply to every generator task
reading it. A spec shared between a jaxrs-spec server and a microprofile client
carries the extension into both, and the client stops compiling.

Reproduced on a minimal spec with all three nullability spellings (3.1 type
union, union + extension, nullable: true + extension): the microprofile field
is a plain String in every case, so openApiNullable cannot rescue it.

Fix

Gate the call sites in microprofile/pojoOverrides.mustache on the same
model-level extension as the definitions, so they can no longer disagree,
falling back to Objects.equals / the bare field name.

This only changes output that previously failed to compile — regenerating all
14 microprofile sample configs produces zero diff.

Tests

New testMicroprofileIgnoresJacksonOptionalNullableExtension_issue24560, which
fails on master and passes with the fix. JavaClientCodegenTest 266 green.

[Ignacio-Vidal]

Opened upstream as OpenAPITools#24560.