[Feature Request]: Add "Open File" action to the export-success notification in SQL Notebooks

### Feature Description

When exporting results from a SQL Notebook cell to CSV, JSON, or Excel, the success toast displays the saved file path but provides no action to open the file. The Query Profiler already solves this with an `Open File` button on its export-success toast. SQL Notebooks should match.

### Current Behavior

After running an export from a SQL Notebook result grid, the toast shows:

> Saved results to `/Users//Untitled-1_resultset_1.json`
> Source: SQL Server (mssql)

No action buttons are available on the notification. The user must navigate to the path manually in their OS file browser to open the file.

### Expected Behavior

The export-success toast should include an `Open File` action (matching the Query Profiler experience) that opens the exported file in VS Code, and ideally also a `Reveal in Finder` / `Show in Explorer` action to open the containing folder.

### Problem and Motivation

SQL Notebooks are explicitly positioned for iterative, interactive analysis. An export almost always leads to a next step (open, inspect, attach, share). Hiding that next step behind manual OS navigation breaks the flow inside VS Code. The Query Profiler already demonstrates the correct pattern in the same extension.

### 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
- [ ] I want to help implement this feature

Locate the SQL Notebook export-success toast and compare it with the Query Profiler export notification, which already has an Open File action. Verify how the exported path is passed to the notification; done means the SQL Notebook toast opens the exported file, with folder reveal behavior resolved if included.

Written by the indexing model from the issue text.