Workspace scoped MCP configuration fail under Windows

## Problem
Workspace-scoped MCP configuration does not work under Windows. Attempting to change a global MCP configuration to a workspace configuration causes an error.

## Steps to reproduce the issue

1. Define any valid Global MCP configuration (gets saved in %USERPROFILE%\.aws\amazonq\mcp.json)
2. Go to VS Code's Amazon Q extension tab
3. Click the MCP server configuration icon
4. Select the MCP server
5. Click Edit Setup
6. Change the scope selection from "Global" to "This workspace"
7. Click Save
8. See error message l;ike: ENOENT: no such file or directory, mkdir 'C:\Users\bmarty\AppData\Local\Programs\Microsoft VS Code\file:\c%3A\landmark\ox3\src\hcm.amazonq\agents'

## Expected behavior
The configuration should be moved from mcp.json to the root of the workspace in a directory called .amazonq. If a file is created in that location it should be picked up.

## Actual Behavior
The extension is apparently failing to combine paths correctly or process the path representing the root of the workspace as a local path leading to an error like:

> ENOENT: no such file or directory, mkdir 'C:\Users\bmarty\AppData\Local\Programs\Microsoft VS Code\file:\c%3A\landmark\ox3\src\hcm.amazonq\agents'

If a file is created in the correct location, it is not honored (no MCP configuration is seen).

## System details

> OS: Windows_NT x64 10.0.22631
> Visual Studio Code extension host: 1.102.2
> Amazon Q: 1.85.0
> node: 22.15.1
> electron: 35.6.0
>

Reproduce the Windows workflow in the VS Code Amazon Q extension, starting with the workspace-scope change from the global mcp.json to the workspace .amazonq directory. Trace how the workspace path is handled and verify that saving creates or updates the workspace configuration and that the extension loads it without the ENOENT error.

Written by the indexing model from the issue text.