chore(vscode): replace deprecated Copilot extension with Copilot Chat (#5662)

# Description of Changes

- **What was changed**  
Updated `.vscode/extensions.json` to replace the deprecated
`GitHub.copilot` extension with `GitHub.copilot-chat`.

- **Why the change was made**  
The original GitHub Copilot extension is deprecated. Using the Copilot
Chat extension ensures continued support, compatibility with current
GitHub tooling, and access to the recommended Copilot experience in VS
Code.

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

.vscode/extensions.json

@@ -13,7 +13,7 @@
     "vscjava.vscode-spring-boot-dashboard", // Spring Boot dashboard for managing and visualizing Spring Boot applications
     "EditorConfig.EditorConfig", // EditorConfig support for maintaining consistent coding styles
     "ms-azuretools.vscode-docker", // Docker extension for Visual Studio Code
-    "GitHub.copilot", // GitHub Copilot AI pair programmer for Visual Studio Code
+    "GitHub.copilot-chat", // GitHub Copilot AI pair programmer for Visual Studio Code
     "GitHub.vscode-pull-request-github", // GitHub Pull Requests extension for Visual Studio Code
     "charliermarsh.ruff", // Ruff code formatter for Python to follow the Ruff Style Guide
     "yzhang.markdown-all-in-one", // Markdown All-in-One extension for enhanced Markdown editing