Expose request ID after token refresh

[najibna]

Description

Fixes #1453

Keeps the existing Session return type and attaches Shopify's x-request-id to the session after a successful refresh. Failed refreshes expose the same ID on HttpResponseError#request_id.

This preserves backward compatibility instead of introducing a new result object.

How has this been tested?

• bundle exec ruby -Itest test/auth/refresh_token_test.rb
• bundle exec srb tc

Checklist:

• My commit message follow the pattern described in here
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the project documentation.
• I have added a changelog line.

[najibna]

I have signed the CLA!