Copilot CLI removes existing content when restructuring a file

### Describe the bug

When asked to add new sections, to an existing structured text file, Copilot replaced the original sections entirely instead of preserving them and appending the new sections alongside. I had to explicitly ask copilot cli to restore the deleted sections. It was able to restore them, thankfully.

### Affected version

GitHub Copilot CLI 1.0.36

### Steps to reproduce the behavior

1. Create a structured text file with multiple entries, each containing a "field X" field with unique content.
2. Ask Copilot CLI to "edit the file to include field a, field b and field c."
3. Review the resulting file for the original "field X" fields.

### Expected behavior

Copilot should add the new sections alongside the existing field in each entry. Th eoriginal fields should remain untouched.

### Additional context

_No response_

Start by reproducing the issue with GitHub Copilot CLI 1.0.36 using a structured text file containing multiple entries and unique "field X" values. Then locate the CLI's file-editing or restructuring path and add coverage showing that new fields are added without removing existing ones; the original fields must remain unchanged.

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