Explorer: consider IdC region when choosing default region

## Problem
When initially logging in (Singing in with Identity Center), I specifically set the region to eu-west-2, but when viewing my credentials in the toolkit extension, they always display us-east-1.

## Steps to reproduce the issue

1. Download extension in VS Code
2. Open the extension
3. For "Choose sign-in option" select "Workforce".
4. Add a valid Start URL, and choose the "eu-west-2" region.
5. Follow the redirects and sign in successfully.
6. Select your newly configured connection (one of your roles)
7. This should display your resources for a configured region. In my case, this is always us-east-1.

## Expected behavior
Step 7 should display resources for the configured region (in my case, eu-west-2), not us-east-1.

## System details (run `AWS: About` and/or `Amazon Q: About`)

- OS: MacOS Sonoma 14.5
- Visual Studio Code version:
Version: 1.90.2 (Universal)
Commit: 5437499feb04f7a586f677b155b039bc2b3669eb
Date: 2024-06-18T22:37:41.291Z
Electron: 29.4.0
ElectronBuildId: 9728852
Chromium: 122.0.6261.156
Node.js: 20.9.0
V8: 12.2.281.27-electron.0
OS: Darwin arm64 23.5.0

- AWS Toolkit version:
v3.11.0

- Amazon Q version:
None

Reproduce the VS Code extension flow from “Choose sign-in option,” selecting Workforce and eu-west-2, then inspect how the Identity Center connection is passed to the resource explorer. Done means the explorer displays resources in the configured eu-west-2 region rather than defaulting to us-east-1.

Written by the indexing model from the issue text.