Replace file tree alert with empty state text

What does this MR do and why?

Replaced the dismissible tip alert with a simple empty state message that appears when no files are included, clearly explaining: "When you add configuration files using include, they appear in this list." Tip alerts are designed to educate users about new features, not to explain empty states—using one here was semantically incorrect. The new approach follows design system guidelines by using the appropriate empty state pattern with clearer, simpler language.

References

Banner Cleanup - Phase 2: Designer Hackathon - ... (gitlab-design#3070 - closed) • Unassigned

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to the Pipeline editor page in a project like /Commit451/lab-coat/-/ci/editor?branch_name=master where pipelines are setup but no files use include
2. Observe the empty state message

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.