Design: SPEP test-run exploration
### Problem Statement
Customers are hesitant to enable Scheduled Pipeline Execution Policies (PEP) due to lack of visibility into the potential impact. They need to understand:
* How many pipelines will be triggered at once
* Estimated execution duration for all pipelines
* Estimated distribution time window
This idea came up in this discussion: https://gitlab.com/groups/gitlab-org/-/epics/17875#note_2597450847
Related issues:
[Spike](https://gitlab.com/groups/gitlab-org/-/epics/17875?show=eyJpaWQiOiI1NTYxMjciLCJmdWxsX3BhdGgiOiJnaXRsYWItb3JnL2dpdGxhYiIsImlkIjoxNzA3MTM3MTZ9#note_2632800548)
### Proposed Solution Overview
Brainstorm [Figma](https://www.figma.com/design/5TBsJus34bYIJ9eETeaRGc/Scheduled-Pipeline-Dry-Run?node-id=0-1&p=f&t=KulPz9p6M8UJazXs-0)
issue