[Feature Request]: SQL Notebooks AI Generate - Multiple Cells

### Feature Description

First off, thank you very much for adding SQL Notebook support to the MSSQL extension.

I see that AI generation has been added, which is awesome.
At the moment, it is a single line text box, although you can paste multiline but you can't hit enter without submitting.

### Problem and Motivation

There seems to be a limitation where the generate button makes a new cell and only the content of that cell is generated.
I would propose that instead of immediately creating a new cell, that you have a larger prompt box, and let the AI potentially create multiple cells.
This would be very helpful for modernizing some old script files.

### 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 tracing the SQL Notebooks AI generation flow in the MSSQL extension, focusing on how the current prompt and generated cell are handled. Determine how multiline prompts and multiple generated cells should be represented and presented. Done means a user can submit a larger prompt and receive more than one notebook cell when appropriate.

Written by the indexing model from the issue text.