[Feature Request]: SQL Notebooks - Multiple connections

### Feature Description

Give me the option to programatically switch the connection context of the notebook

### Problem and Motivation

Currently, a notebook can only have one connection - I am fine with that
That connection can be changed - also perfect.
If I need different cells to run against different connections, i have to run the notebook cell by cell though.
I would like to have a proper runbook where I can run all - to facilitate that, I would need to either have multiple connections at the same time or some code snippet that allows me to programatically switch my connection in a cell

### 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

SQL Notebooks

### Confirmation

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

Start by locating the SQL Notebooks implementation and understanding how the current single connection is selected and changed. Define how notebook cells will select different connections or switch context programmatically, then verify that a complete notebook can run all cells against their intended connections.

Written by the indexing model from the issue text.