Add Confidential Attribute to CreateTestCase Mutation

What does this MR do and why?

Relates to #422117 (closed)

As part of &11229 (closed), we will be adding support for creating and editing confidential test cases in the UI.

While test cases will be migrated to the work items framework mid-next year, this is an interim solution to help enable the Quality team to manage test cases within a single source of truth for ongoing confidential projects.

This MR updates the CreateTestCase GraphQL mutation to accept a confidential attribute, which will eventually be used by Add Confidentiality Checkbox to New Test Case Form (#422121 - closed). This mutation is currently used by the frontend when creating new test cases in a project. The underlying service used by this mutation is Issues::CreateService, which already supports confidentiality.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Creating a Test Case via UI:

1. Check out the branch for this MR to your local machine
2. Ensure you have an Ultimate license available on your GDK
3. Create a new group and project
4. In the project, go to Build > Test cases
5. Click on the "New test case" button
6. Create a new test case
7. Verify no regressions occur and test case will currently save as non-confidential

Creating a Test Case via GraphQL:

1. Check out the branch for this MR to your local machine
2. Login as a member with reporter access or above for the project you previously created
3. Go to http://<your gdk url>/-/graphql-explorer
4. Add the following for the mutation:

mutation createTestCase($createTestCaseInput: CreateTestCaseInput!) {
  createTestCase(input: $createTestCaseInput) {
    errors
    testCase {
      confidential
    }
  }
}

5. Add the following for the query variables:

{
  "createTestCaseInput": {
    "projectPath": "<full path of your project, ex: group-a/project-a>",
    "confidential": true,
    "description": "Test confidentiality",
    "title": "Confidential Test"
  }
}

6. Click the execute "play button" to send the mutation
7. Check in your project test cases and verify you have a confidential test case (please note, the confidential icon next to the test case title will be added in #422119 (closed))
8. Now login as a user who is not a project member or is a guest of the project
9. Attempt to view the test case, verify you are taken to a 404 page

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.