Adds variation by the header name Accept-Language to the delivery API output cache policy

[AndyButland]

Prerequisites

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

Resolves: #19706

Description

The delivery API output cache isn't varying over the accepted headers, leading to the problem described in the linked issue.

This PR adjusts the policy to vary over the headers that could be used for the document and media requests are included in the collection of details to vary by.

Note that "Preview" isn't necessary to include as preview requests aren't cached anyway.

Testing

See the linked issue. Note that once the PR is applied the request for the same item by different languages is not cached such that each language can be retrieved.

Confirm that output caching still works by modifying the content item and note that the old values are retrieved until the cache time expires.

Merge and Release

When merged this will be part of 13.10, but should also be cherry-picked up for 16.2.

When porting to 16, include "Accept-Segment" in the constants.

[Copilot]

Pull Request Overview

This PR enhances the Delivery API’s output caching by varying cached responses based on the Accept-Language and Start-Item headers, ensuring different languages don’t share the same cache entry.

• Added StringValues import and new constructor to accept header names in DeliveryApiOutputCachePolicy
• Updated AddOutputCache to pass header names for content and media caching policies
• Modified CacheRequestAsync to apply the header-based vary rules

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/Umbraco.Cms.Api.Delivery/DependencyInjection/UmbracoBuilderExtensions.cs	Imported StringValues and updated AddOutputCache to pass header names for both content and media caching policies
src/Umbraco.Cms.Api.Delivery/Caching/DeliveryApiOutputCachePolicy.cs	Introduced a new constructor overload accepting StringValues and applied the header names to CacheVaryByRules.HeaderNames in CacheRequestAsync

Comments suppressed due to low confidence (2)

src/Umbraco.Cms.Api.Delivery/DependencyInjection/UmbracoBuilderExtensions.cs:112

• Consider adding a unit or integration test that sends requests with different Accept-Language headers to verify the output cache varies as expected and does not return the same cached response.

                options.AddPolicy(Constants.DeliveryApi.OutputCache.ContentCachePolicy, new DeliveryApiOutputCachePolicy(outputCacheSettings.ContentDuration, new StringValues(["Accept-Language", "Start-Item"])));

src/Umbraco.Cms.Api.Delivery/DependencyInjection/UmbracoBuilderExtensions.cs:112

• The array literal syntax new StringValues(["Accept-Language", "Start-Item"]) is invalid C#. Use either new StringValues(new[] { "Accept-Language", "Start-Item" }) or the params-based constructor: new StringValues("Accept-Language", "Start-Item").

                options.AddPolicy(Constants.DeliveryApi.OutputCache.ContentCachePolicy, new DeliveryApiOutputCachePolicy(outputCacheSettings.ContentDuration, new StringValues(["Accept-Language", "Start-Item"])));

[AndyButland]

Merged for 13 and cherry-picked over to main with 4deb756 for 16.