Fix issue with preview in delivery API for MNTP property editor

[AndyButland]

Prerequisites

• I have added steps to test this contribution in the description below

Resolves: #19630

Description

The linked issue shows that when requesting a document in preview via the delivery API, documents picked via a MNTP that aren't published aren't included in the output.

It looks like we just aren't passing on the preview flag in the property value converter, so I've done that, added a unit test and adjusted the mocks to accommodate the changes so the other unit tests continue to pass.

Testing

See linked issue and create a setup like the following.

Request the first page via the delivery API:

GET /umbraco/delivery/api/v2/content/item/{id}
Preview: true
Api-Key: {api key}

Note that after this PR the property details include the second page.

[Copilot]

Pull Request Overview

This PR ensures the preview flag is propagated through the MultiNodeTreePicker delivery API converter and updates tests to cover draft items.

• Passes the preview parameter into IPublishedContentCache.GetById calls in MultiNodeTreePickerValueConverter
• Adds a new unit test for preview mode in single-node MNTP scenarios
• Updates mocks and helper methods in test base to handle the new preview overload

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/Umbraco.Core/PropertyEditors/ValueConverters/MultiNodeTreePickerValueConverter.cs	Use GetById(preview, id) instead of always calling the non-preview overload
tests/.../PropertyValueConverterTests.cs	Introduce DraftContent and configure preview-aware cache mock setups
tests/.../OutputExpansionStrategyTestBase.cs	Change RegisterContentWithProviders invocation to include false preview flag
tests/.../MultiNodeTreePickerValueConverterTests.cs	Add a preview-mode test and update cache setup calls

Comments suppressed due to low confidence (1)

tests/Umbraco.Tests.UnitTests/Umbraco.Core/DeliveryApi/MultiNodeTreePickerValueConverterTests.cs:108

• The test references DraftContent, but this property isn't defined in MultiNodeTreePickerValueConverterTests. You should create a draft content mock within this test (e.g., via SetupPublishedContent) and register it for preview mode, then use that instance instead of DraftContent.

        var inter = new Udi[] { new GuidUdi(Constants.UdiEntityType.Document, DraftContent.Key) };

[nikolajlauridsen]

Looks good, tests good 👍