[Feature Request]: Notify or Merge when Schema Designer is stale due to concurrent edits

@aasimkhan30 is already working on this.

Since Mar 4, 2026.

### Feature Description

Notify the user when their view is out-of-date, and provide options to update their view, compare changes (potentially using Copilot or a diff engine), and merge to resolve conflicts. The workflow should be similar to a CI/CD flow, handling schema comparison and merges to streamline collaboration.

**Design mock:**

**Entry Point**

- Show a persistent Out of Sync indicator in the Schema Designer toolbar/status area once drift is detected.
- The indicator must remain visible until the drift is resolved (sync/merge/discard).
- Provide a discoverable action in the Copilot chat (e.g, “Check if the database schema is in sync”)
- Drift notification modal
- Refresh & Sync (primary): reload the remote schema and rebase the local design (see behavior below)
- Compare: open a diff view between local design snapshot and remote schema (see compare requirements)
- Discard Local: discard local in-designer changes and align to remote

**Compare diffs**

- Provide a Schema Diff view with:
- side-by-side and unified modes
- a summary header (e.g., “X additions, Y modifications, Z deletions”)
- color highlights

### Problem and Motivation

When using the Schema Designer, if the schema has been changed by other users after initial load, the diagram only reflects the original state and becomes stale. If the user publishes their changes, it may result in errors due to conflicts or outdated schema. To address this, introduce a mechanism to detect if the schema is outdated because of concurrent edits. With a notification and merge system, users would be aware of concurrent edits, able to update or merge their schemas, and reduce the risk of publishing outdated or conflicted changes.

### 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
- [x] Schema Designer
- [ ] Schema Compare
- [ ] Local SQL Server Container provisioning
- [ ] SQL database in Fabric provisioning
- [ ] DACPAC/BACPAC export/import
- [ ] 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
- [ ] I want to help implement this feature