Variables don't work in collection authentication (OAuth 2)

[joshtransient]

Describe the bug
Variables don't resolve when using OAuth 2.0 as the authentication method, and client credentials as the grant type. URLs and the body just use the variable names when Fetch Client tries to request tokens.

To Reproduce
Steps to reproduce the behavior:

1. Create a custom set of variables (Variable > ... > New Variable) and add some entries
2. Create a collection (Collection > … > New collection)
3. Attach the variable set (name of collection] > … > Attach Variable)
4. Edit the collection, update its Authentication method to OAuth 2.0 with grant type Client Credentials
5. Fill in values for token URL, client ID, secret, etc using variables
6. Open the Fetch Client output to observe the URLs being called
7. Click [Request New Token]
8. The Fetch Client output should show that it is not resolving the variables

Expected behavior
Variables should resolve to their correct values before trying to request a new token.

Screenshots
Fetch Client output from secondary side bar is on the left, the collection is on the right:

Desktop (please complete the following information):

• OS: macOS 15.1
• Fetch Client Version: 1.8.0
• VS Code Version: 1.95.0
• Node Version: ??? (I installed an extension, does this require node to be installed on this machine?)

Additional context
N/A