fix(vscode): update the default config for endpoint.

[icycodes]

Update default configuration for endpoint:
From: "" (Empty string, which uses the configuration from the agent config file)
To: "http://localhost:8080" (Endpoint and token will be saved in VSCode settings)

This change improves the user experience for users who start the Tabby server at localhost:8080, then launch VSCode and install the Tabby extension. The status bar will prompt the user to update the token.

Before the change: Update token opens the agent config file.
After the change: Update token shows an input prompt for the token, which is then saved in VSCode settings.