Validate URL when pull mirroring via UI

What does this MR do and why?

Validates URLs during pull mirroring config via the UI, ensuring consistency with API behavior and preventing invalid URLs from being saved.

Previously, the UI accepted any pull mirror URL without validation, only failing later during the actual mirror sync job. This created a poor user experience where users wouldn't discover URL issues until after saving the config. The API, however, validates URLs upfront using Import::ValidateRemoteGitEndpointService.

This MR ensures creation of pull mirrors route through Repositories::PullMirrors::UpdateService, which validates URLs before saving.

References

Use `ValidateRemoteGitEndpointService` for pull... (#419644)

Screenshots or Screen recordings

Before

Screen_Recording_2025-12-07_at_02.23.55

After

Screen_Recording_2025-12-07_at_02.28.07

How to set up and validate locally

Check reproduction steps in the issue, an error message should be displayed at the top

MR acceptance checklist

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