FE: Use real agent-mapping API in Create Workspace UI

Issue: FE: Replace mock group-agent mapping GraphQL wi... (#455214 - closed)

What does this MR do and why?

• Removes the @client annotation from the agent-mapping GraphQL query.
• Unskip the feature spec that exercises the "Create Workspace" UI when the remote_development_namespace_agent_authorization feature flag is on.
• Rewrites the message that appears when there aren't cluster agents available to create a workspace for a given project.

MR acceptance checklist

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Screenshots or screen recordings

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Before	After

How to set up and validate locally

Given the complexity of setting up remote development in a local environment, I recommend testing these changes by running the feature tests locally using CHROME_HEADLESS=0 flag. Follow these instructions to use this approach:

1. Apply the following patch. It will insert a debugging breakpoint in the feature spec.

   diff --git a/ee/spec/features/remote_development/workspaces_spec.rb b/ee/spec/features/remote_development/workspaces_spec.rb
   index 9916870ef2c2..e8daba5e45ce 100644
   --- a/ee/spec/features/remote_development/workspaces_spec.rb
   +++ b/ee/spec/features/remote_development/workspaces_spec.rb
   @@ -50,6 +50,8 @@
    
          # CREATE WORKSPACE
    
   +      live_debug
   +
          click_link 'New workspace', match: :first
          click_button 'Select a project'
          find_by_testid("listbox-item-#{project.full_path}").click
   

2. Run the feature spec using the following command: CHROME_HEADLESS=0 bundle exec rspec ee/spec/features/remote_development/workspaces_spec.rb

3. When the Chrome Browser opens, click New Workspace

4. Select a project in the dropdown.

5. Select a cluster agent.

6. Click "Create workspace".

7. A workspace should be created.