FE: Replace mock group-agent mapping GraphQL with real API

MR: FE: Use real agent-mapping API in Create Worksp... (!150791 - merged)

Description

After delivering FE: Use group-agent mapping API in Create Works... (#444860 - closed) and BE: Implement new query API with filtering supp... (#441555 - closed), we integrate the Workspace Creation UI with the new cluster agents query API.

Acceptance Criteria

• Replace mocked GraphQL API with production ready GraphQL API.
• Enable feature spec that exercises the Create Workspace UI using the new GraphQL API.