Fix VTA SSO authorize host for tenant subdomains

[ethriel3695]

Summary

• route the VTA SSO OAuth authorize request through the tenant subdomain instead of always using www.chromatic.com
• reuse the same subdomain rewrite for the verification URL so the SSO device flow stays on the same tenant host
• add a regression test covering both the SSO and default sign-in paths

How to QA

1. Run yarn vitest run src/utils/requestAccessToken.test.ts --reporter=verbose.
2. In Storybook while signed out of the Visual Tests addon, open the panel and click Sign in with SSO.
3. Enter an SSO tenant subdomain such as storybook and continue.
4. Complete the SSO flow in the popup and confirm the addon proceeds to project selection or the linked-project state instead of showing a failed authentication/projects error.
5. Repeat the standard Sign in with Chromatic flow and confirm the non-SSO path still uses the default host successfully.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 6.90%. Comparing base (28e7c37) to head (06f2772).

Additional details and impacted files

@@           Coverage Diff            @@
##            next    #423      +/-   ##
========================================
+ Coverage   6.50%   6.90%   +0.40%     
========================================
  Files        166     166              
  Lines      14164   14164              
  Branches     305     316      +11     
========================================
+ Hits         921     978      +57     
+ Misses     13163   13109      -54     
+ Partials      80      77       -3     

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[ethriel3695]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Merge conflicts resolved in commit 1befaf1. The branch was behind origin/main by several commits (version bumps to 5.1.2, CHANGELOG updates, and a sidebar fix). The merge was clean — no actual conflicts in the changed files (requestAccessToken.ts / requestAccessToken.test.ts). All tests still pass.