[Bug]: Improve SQL Notebooks quality and performance

[croblesm]

Description

NPS feedback ahead of GA surfaced several quality and performance issues with SQL Notebooks that need to be addressed before release.

Issues reported from NPS:

• Runtime instability: Kernel intermittently switches from MSSQL to Python, and results don't always render correctly.
• Unclear connection state: Noticeable lag when connecting via notebook with no clear indicator of connection status (standard queries are fine).
• Errors not surfaced: Failed queries in notebooks don't show error messages.
• Weak IntelliSense: Completions/suggestions in notebook cells are not helpful.

Proposed fixes:

• Kernel stays on MSSQL and does not switch unexpectedly
• Results render reliably across cells
• Clear connection state indicator; notebook connection latency on par with standard editor
• Query errors surfaced in cell output
• IntelliSense reaches parity with the standard SQL editor

Steps to Reproduce

1. Open the MSSQL extension
2. Connect to a database
3. Create a new SQL Notebook

Affected 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
• Other (please describe below)

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

SQL Notebooks

Environment Information

• VS Code: Latest
• MSSQL extension: Latest

Confirmation

• I have searched existing issues and couldn't find a match
• I want to work on this issue