Fix re-use of extensions between instances

What does this MR do and why?

The MR fixes the bug introduced in the recent Source Editor refactoring when editing files in MR was throwing JS errors.

Screenshots or screen recordings

Before	After
77319-before-xs	77319-after-xs

How to set up and validate locally

1. Open an MR in the Web IDE
2. Wait for the Web IDE to load and display the changed files
3. Switch to the Edit tab by clicking the </> icon
4. The files should be ready for editing without any error

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #348998 (closed)