Sidebar add to-do for alert (Alert todo create mutation)

What does this MR do?

User Overview

This adds a graphql mutation to mark an alert as a todo in the sidebar of alert management.

Not in scope: 'Mark as Done' button appearing for alerts which have todos already (!34789 (merged) will cover that).

Related issues:

• #215946 (closed)
• #222359 (closed)

Technical Overview

Example curls:

No existing todo:

curl 'https://23013.qa-tunnel.gitlab.info/api/graphql' --header "Authorization: Bearer $GRAPHQL_TOKEN" --header "Content-Type: application/json" --request POST --data '{"query": "mutation {alertTodoCreate(input: {projectPath: \"gitlab-org/gitlab-test\",iid: \"4\"}) {alert {iid}, todo {id},errors}}"}'
{"data":{"alertTodoCreate":{"alert":{"iid":"4"},"todo":{"id":"gid://gitlab/Todo/620"},"errors":[]}}}

Existing ‘marked’ todo for that alert/user:

curl 'https://23013.qa-tunnel.gitlab.info/api/graphql' --header "Authorization: Bearer $GRAPHQL_TOKEN" --header "Content-Type: application/json" --request POST --data '{"query": "mutation {alertTodoCreate(input: {projectPath: \"gitlab-org/gitlab-test\",iid: \"4\"}) {alert {iid}, todo {id},errors}}"}'
{"data":{"alertTodoCreate":{"alert":{"iid":"4"},"todo":null,"errors":["Existing pending todo for alert and user"]}}}

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team