Add relatedMergeRequests field definition and resolver to Workspace GraphQL type

MR: Pending

Description

As part of our new designs for Workspaces, we need the frontend to display merge request information in expanded workspace cards, and for that we need to add a relatedMergeRequests field to the existing Workspace GraphQL type and implement its resolver.

Acceptance criteria

• Add relatedMergeRequests field definition to existing Workspace GraphQL type
• Implement resolver method that fetches open merge requests on the same branch from the same project
• Return relevant MR fields needed for the designs
• Add proper authorisation checks to ensure user can view the merge requests
• Add error handling for cases where project or MR data is unavailable
• Update GraphQL field documentation with examples

TODO: Implementation plan