[Feature Request]: scmp file integration with sqlproj explorer

### Feature Description

Open Schema Comparison (scmp) files directly from database project or explorer view instead of having to open the Schema Comparison tool then open the file.

### Problem and Motivation

I am using the new SDK-based SQL projects. If I include a Schema Compare file (scmp) in my project, there's not an easy way to open that Schema Comparison. Clicking on the file just opens up the raw XML file in the editor. Right now, the flow is that I have to right click on my project, click "Schema Compare (Preview)" then once the Schema Compare window opens I have to click "Open .scmp file" and browse to the right location.

It would be really helpful to be able to just click on the schema compare file in the project and have it open the comparison window.

### Related Area

- [ ] Connection dialog (SQL Server | Azure browse/Fabric browse)
- [ ] Query editor
- [ ] Query results panel
- [ ] Object Explorer
- [ ] GitHub Copilot integration
- [ ] Preview/Edit data
- [ ] Table Designer
- [ ] Schema Designer
- [x] Schema Compare
- [ ] Local SQL Server Container provisioning
- [ ] SQL database in Fabric provisioning
- [ ] DACPAC/BACPAC export/import
- [x] SQL Database projects
- [ ] Query Plan Visualizer
- [ ] Other (please describe below)

### If you selected "Other", please describe the area

_No response_

### Confirmation

- [x] I have searched existing feature requests and couldn't find a match
- [x] I want to help implement this feature

Start by tracing how the SQL Database Projects explorer handles .scmp files and how the Schema Compare (Preview) entry point opens them. Compare the current raw-XML behavior with the existing Open .scmp file flow. Done means clicking an included .scmp file from the project or explorer opens the Schema Comparison window instead of the XML editor.

Written by the indexing model from the issue text.