Validate remote mirror url uniqueness and self-reference

What does this MR do and why?

Contributes to #603810 (closed)

Problem

The RemoteMirror model did not prevent a project from adding multiple push mirrors with the same URL, nor from pointing a mirror at the project's own repository. Both were silently accepted, leading to redundant sync jobs and self-mirroring loops.

Solution

• Add a create-only url_not_duplicated validation that rejects a second mirror with the same sanitized URL on the same project.
• Add a create-only url_not_pointing_to_self validation that rejects a mirror targeting the project's own HTTP or SSH repository URL.
• Compare on the sanitized stored URL so credentials neither leak into errors nor make detection credential-sensitive.
• Scope both to on: :create so existing duplicate or self-mirrors are grandfathered and ongoing syncs, toggles, and edits keep working.

References

• Issue: #603810 (closed)
• MR that surfaced this: !240866 (merged)
• Discussion thread: !240866 (comment 3482834754)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to a project's Settings > Repository > Mirroring repositories.
2. Add a push mirror with some URL, then try adding a second mirror with the same URL — it is now rejected.
3. Try adding a mirror pointing at the project's own repository URL — it is now rejected.
4. Run the specs: bin/rspec spec/models/remote_mirror_spec.rb.

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