[Feature Request]: Add API diagram to improve readability of config preview

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### Feature Description

When users click Show Config, we already display a DAB configuration preview before deployment. Add an additional tab called API Diagram that visualizes the generated API surface (REST endpoints, GraphQL operations, MCP tools) derived from the current configuration.

**Design Mock:**

**Functional requirements:**
- A new API Diagram tab is available in Show Config alongside existing config preview.
- REST section lists the base path and the computed REST endpoints for enabled entities.
- GraphQL section lists the GraphQL endpoint and available queries/mutations derived from enabled entities/mutations
- MCP section lists MCP endpoint and shows enabled/disabled tools derived from permissions, plus a scope summary (e.g., “2 entities exposed, operations enabled: Read/Create”).

### Problem and Motivation

The DAB config file is powerful but not always easy to interpret quickly, especially for users who want to understand:
- What endpoints will be created (and their base paths)
- What GraphQL queries/mutations will be available
- What MCP tools are enabled/disabled based on exposed entities and permitted operations

A diagram-style view provides a fast “API contract” overview and reduces misconfiguration surprises.

### 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
- [ ] Schema Compare
- [ ] Local SQL Server Container provisioning
- [ ] SQL database in Fabric provisioning
- [ ] DACPAC/BACPAC export/import
- [ ] SQL Database projects
- [ ] Query Plan Visualizer
- [x] Other (please describe below)

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

DAB

### Confirmation

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

Start by tracing the existing Show Config UI and its configuration-preview data flow. Add an API Diagram tab covering REST endpoints, GraphQL operations, and MCP tools derived from the current configuration; done means the three sections show the requested endpoints, operations, permissions, and scope summary alongside the preview.

Written by the indexing model from the issue text.