Fix creating release asset links when using the API

When creating relase with asset links using the API, `link_type` and
`filepath` were not declared as params and thus not set.

This adds them as optional declared params to the
`POST /projects/:id/releases` endpoint.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/230698.

changelogs/unreleased/230698-fix-create-release-asset-link.yml

+---
+title: Fix creating release asset links when using the API
+merge_request: 37557
+author:
+type: fixed

lib/api/release/links.rb

@@ -40,7 +40,7 @@ module API
               requires :name, type: String, desc: 'The name of the link'
               requires :url, type: String, desc: 'The URL of the link'
               optional :filepath, type: String, desc: 'The filepath of the link'
-              optional :link_type, type: String, desc: 'The link type'
+              optional :link_type, type: String, desc: 'The link type, one of: "runbook", "image", "package" or "other"'
             end
             post 'links' do
               authorize! :create_release, release

lib/api/releases.rb

@@ -50,8 +50,10 @@ module API
         optional :ref,         type: String, desc: 'The commit sha or branch name'
         optional :assets, type: Hash do
           optional :links, type: Array do
-            requires :name, type: String
-            requires :url, type: String
+            requires :name, type: String, desc: 'The name of the link'
+            requires :url, type: String, desc: 'The URL of the link'
+            optional :filepath, type: String, desc: 'The filepath of the link'
+            optional :link_type, type: String, desc: 'The link type, one of: "runbook", "image", "package" or "other"'
           end
         end
         optional :milestones, type: Array[String], coerce_with: ::API::Validations::Types::CommaSeparatedToArray.coerce, desc: 'The titles of the related milestones', default: []

spec/requests/api/releases_spec.rb

@@ -420,7 +420,17 @@ RSpec.describe API::Releases do
       {
         name: 'New release',
         tag_name: 'v0.1',
-        description: 'Super nice release'
+        description: 'Super nice release',
+        assets: {
+          links: [
+            {
+              name: 'An example runbook link',
+              url: 'https://example.com/runbook',
+              link_type: 'runbook',
+              filepath: '/permanent/path/to/runbook'
+            }
+          ]
+        }
       }
     end
 
@@ -435,9 +445,17 @@ RSpec.describe API::Releases do
         post api("/projects/#{project.id}/releases", maintainer), params: params
       end.to change { Release.count }.by(1)
 
-      expect(project.releases.last.name).to eq('New release')
-      expect(project.releases.last.tag).to eq('v0.1')
-      expect(project.releases.last.description).to eq('Super nice release')
+      release = project.releases.last
+
+      aggregate_failures do
+        expect(release.name).to eq('New release')
+        expect(release.tag).to eq('v0.1')
+        expect(release.description).to eq('Super nice release')
+        expect(release.links.last.name).to eq('An example runbook link')
+        expect(release.links.last.url).to eq('https://example.com/runbook')
+        expect(release.links.last.link_type).to eq('runbook')
+        expect(release.links.last.filepath).to eq('/permanent/path/to/runbook')
+      end
     end
 
     it 'creates a new release without description' do