Refactor: Extract Frontend Request Checking Logic

[StarkOne]

This PR refactors the EnsureFrontendRequestsAreStateful middleware to improve its structure and adhere more closely to the Single Responsibility Principle.

Description

The EnsureFrontendRequestsAreStateful middleware was previously responsible for two distinct tasks:

1. Determining if a request originated from a stateful frontend domain.
2. Applying the appropriate middleware pipeline based on that determination.

This refactor extracts the domain-checking logic into its own dedicated class, FrontendRequestChecker. This simplifies the middleware, making its sole responsibility to orchestrate the middleware pipeline.

Changes Made

• New Class: Added a new Laravel\Sanctum\Http\Middleware\FrontendRequestChecker class.
• Logic Extraction: Moved the logic for parsing referer/origin headers and comparing them against the stateful configuration from EnsureFrontendRequestsAreStateful::fromFrontend() to the new FrontendRequestChecker::isFromFrontend() method.
• Middleware Simplification: The EnsureFrontendRequestsAreStateful middleware now calls the new checker class. Its internal static method fromFrontend() has been removed.

Benefits of this Change

• Improved Readability: Each class now has a clear and focused purpose.
• Single Responsibility Principle: The middleware is no longer concerned with the implementation details of how a request is identified as coming from the frontend.
• Enhanced Testability: The FrontendRequestChecker can now be unit-tested in complete isolation, leading to simpler and more robust tests.

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!