Restore _convert_item_to_dict Functionality in ItemValidationPipeline for Compatibility

[arslansherazi]

This pull request restores the _convert_item_to_dict functionality in the ItemValidationPipeline, which was available in previous versions but removed in the current release.

For users who extended the ItemValidationPipeline with the spidermen pipeline and relied on the
_convert_item_to_dict function, this removal could break their code. By reintroducing this functionality, we ensure those users can continue using it without needing to refactor their pipelines.

This update provides compatibility for users relying on the older version's behavior, while allowing users of the current version to avoid any issues or breaking changes.

Sample Code:

[curita]

issue: item_adapter is needed later for self._add_errors_to_item(item_adapter, errors) below.

suggestion: Maybe a test case could be added to check that case.

That's probably why _convert_item_to_dict() was removed. We needed the item adapter later, which that method discarded otherwise.

I think the removal of _convert_item_to_dict() isn't necessarily a backward-incompatible change, as developers shouldn't depend on internal methods. We could restore it, but this pipeline won't use it (as it needs a reference to the adapter object). Or it might use it with some modifications, but at that point, developers will have to refactor their ItemValidationPipeline extensions to accommodate those modifications, too.

@arslansherazi, could you show us some examples of how this pipeline was extended so we can see how to support that case?