Fixes #4076

Generated mapper implementations dropped the JSpecify @Nullable annotations declared on the mapper method they implement, so IDEs reported "Overriding method parameters are not annotated" on the generated @Override methods.

The nullability information was already resolved for null-check decisions (NullabilityResolver), but was never emitted into the generated signature. This propagates it:

• Parameter and MappingMethod now carry the type of the @Nullable annotation to emit, and include it in their import types so the annotation is imported rather than fully qualified.
• AbstractMappingMethodBuilder#propagateNullability resolves the nullability of the overridden method's return type and parameters via the existing MappingBuilderContext#getNullabilityInMapperScope, and is called from BeanMappingMethod.Builder and ContainerMappingMethodBuilder (covering bean, iterable, map and stream mapping methods).
• The corresponding FTL templates emit the annotation inline, directly in front of the return type / parameter type.

Since JSpecify's @Nullable and @NonNull are TYPE_USE-only annotations, they are not valid in method-declaration position and therefore cannot go through the existing declaration-level annotations list.

Design note

Only an explicit @Nullable is reproduced. @NonNull is deliberately left out: inside a @NullMarked scope it is the implied default, so emitting it would add noise to every generated signature without changing the contract. Happy to change this if you'd prefer @NonNull to be explicit as well.

Nothing is propagated for forged methods, which do not override a mapper method. All behaviour stays gated behind the existing mapstruct.disableJSpecify option through the resolver.

Testing

• New tests in org.mapstruct.ap.test.bugs._4076 covering @Nullable on parameters and return types, the generated import, and the container/update method paths (which route through ContainerMappingMethodBuilder rather than BeanMappingMethod.Builder).
• Six existing JSpecify fixtures were updated: each declares @Nullable on the affected parameter, so the added annotation is the corrected output.
• Full processor suite passes: 3653 tests, 0 failures, on both javac and ECJ. Checkstyle and the package-cycles enforcer also pass.