Expression editor tokenizes string template delimiters

[kanushka]

Description

The new expression editor incorrectly tokenizes the string template delimiters ${ and } as clickable/editable tokens when a string template is added (e.g., `sample text is ${var1}.`).

This allows users to modify these delimiters, breaking the string template syntax and causing runtime errors.

Steps to Reproduce

1. Open the new expression editor.
2. Add a string template containing a variable, e.g., `sample text is ${var1}.`
3. Observe that the ${ and } characters are highlighted or treated as editable tokens.
4. Click on ${ or } and change its value (e.g., change } to )).
5. Observe that the code is now broken.

Screen.Recording.2025-11-06.at.23.31.35.mov

Suggested Fix

Render the string template delimiters ${ and } visually differently (e.g., grayed out or plain text) and make them non-clickable/non-editable to maintain code integrity. Only the variable name inside the brackets (var1 in this example) should remain an editable token.

[kanushka]

The current expression editor string template implementation fixes this.
wso2/vscode-extensions#893