[Issue] Move SetGuestEmailOnCart logic in a separated class

[m2-assistant]

This issue is automatically created based on existing pull request: #40395: Move SetGuestEmailOnCart logic in a separated class

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Description (*)

This PR refactors the setGuestEmailOnCart GraphQL mutation by extracting business logic from the resolver into a dedicated model class, improving code organization and testability.

Changes

• New Model Class: Created Magento\QuoteGraphQl\Model\Cart\SetGuestEmailOnCart to handle the business logic of setting guest email on cart
• Refactored Resolver: Updated Magento\QuoteGraphQl\Model\Resolver\SetGuestEmailOnCart to delegate business logic to the new model class
• Unit Tests: Added comprehensive unit test coverage for the new model class (SetGuestEmailOnCartTest)

Motivation

The objective of this modification is to be able to more easily add logic (such as a custom control) before saving the email.

Today, the only solution is to make a beforeExecute plugin on the SetShippingMethodsOnCart class without benefiting from the controls already in place.

Manual testing scenarios (*)

Execute the graphQL setGuestEmailOnCart mutation, it must not have an error.

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• README.md files for modified modules are updated and included in the pull request if any README.md predefined sections require an update
• All automated tests passed successfully (all builds are green)