[Bug]: Copying SQL text in notebook editor cell causes an exception in console

### Description

When copying a SQL text segment in a notebook cell with Ctrl-C an error is generated in the console log.

```
[Error - 12:22:15 PM] Request textDocument/definition failed.
Message: Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'index')
Code: 0
```

### Steps to Reproduce

1. Open SQL Notebook and type some SQL
2. Select the text and type Ctrl-C

Expected: Selected text is copied without error

Actual: An error message is displayed.

Note: This doesn't repro 100% of time, so it seems related to some other state in the cell/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

_No response_

### Environment Information

Latest

### Confirmation

- [x] I have searched existing issues and couldn't find a match
- [ ] I want to work on this issue

Start by reproducing the Ctrl-C action in a SQL Notebook cell and capture the textDocument/definition error shown in the console. Trace the notebook editor and definition-request path involved in copying selected SQL text. Done means copying succeeds without the index-out-of-range exception, including the intermittent state described in the issue.

Written by the indexing model from the issue text.