Add class for 2026-07 REST API support

[aovertus]

Description

Please, include a summary of what the PR is for:

• 2026-07 has been introduced in Add support for 2026-04 API version #1442 but is missing the API classes. Using 2026-07 with the gem will lead to ``uninitialized constant * (NameError)

How has this been tested?

Tested locally

Checklist:

• My commit message follow the pattern described in here
• I have performed a self-review of my own code.
  • 2026-04 folder has been duplicated and find-replace was performed to change the version
• I have added tests that prove my fix is effective or that my feature works.
  • Duplicated 2026-04 test folder
• I have updated the project documentation.
• I have added a changelog line.

[aovertus]

I have signed the CLA!

[aovertus]

Hi @lizkenyon , @byrichardpowell, if there is any chance we could get this released ASAP.

2026-07 upgrade was raised by Shopify support following one of our request as our app (using 2026-04) is unable to find and fetch some shopify collections right now