Further fix to configuration classes using ISet, resolving regression with custom 404 pages

[AndyButland]

Prerequisites

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

Resolves: #19567

Description

The cause of this issue was another cases of issues involved in the fix #19229, where ISet in configuration classes aren't bound.

Note that I've added a compatibility suppression for the breaking change, which I don't see how we can otherwise avoid.

Testing

Prepare a custom 404 page as per the instructions in the documentation (though note that I found this didn't work when using the name "404" for the document type and template, and instead used "File Not Found"). With this PR in place, the custom 404 should be displayed as expected.

To check via debugging, put a breaking point at the start of NotFoundHandlerHelper.GetCurrentNotFoundPageId and verify the error404Collection parameter matches the configured value.

Release

Can consider for a 16.0.1, otherwise 16.1.0.

[Copilot]

Pull Request Overview

This PR fixes the binding regression for custom 404 pages by changing the configuration property type and updating affected tests, and suppresses a compatibility warning.

• Changed Error404Collection from ISet<ContentErrorPage> to IEnumerable<ContentErrorPage> with a C# 12 collection expression default.
• Updated unit test initializer syntax in ContentSettingsValidatorTests.cs to use [...].
• Added a compatibility suppression for the breaking change in CompatibilitySuppressions.xml.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/Umbraco.Tests.UnitTests/Umbraco.Core/Configuration/Models/Validation/ContentSettingsValidatorTests.cs	Switched collection initializer syntax to C# 12 [...]
src/Umbraco.Core/Configuration/Models/ContentSettings.cs	Changed property type from ISet to IEnumerable
src/Umbraco.Core/CompatibilitySuppressions.xml	Added suppression for CP0002 on Error404Collection

Comments suppressed due to low confidence (2)

src/Umbraco.Core/Configuration/Models/ContentSettings.cs:53

• Changing Error404Collection from ISet to IEnumerable removes set semantics and mutability; consider using IReadOnlySet or IReadOnlyCollection to preserve uniqueness and clearly express intended behavior.

public IEnumerable<ContentErrorPage> Error404Collection { get; set; } = [];

src/Umbraco.Core/Configuration/Models/ContentSettings.cs:50

• [nitpick] The XML doc still implies set behavior; update the summary to reflect the change to an IEnumerable and clarify whether duplicates are permitted.

///     Gets or sets a value for the collection of error pages.