Support subfolder-based installations for internal route detection

Problem/Motivation

The entity_mesh module fails to correctly detect internal routes in Drupal installations
that are hosted in subfolders. As a result, valid routes (e.g., /article accessed via
https://localhost/subfolder/article) are marked as broken.

Steps to reproduce

1. Install Drupal in a subfolder (e.g., /subfolder).
2. Enable entity_mesh.
3. Add a valid internal route, such as https://localhost/subfolder/home on a content.
4. Access to the entity mesh report /admin/reports/entity-mesh/table.
5. Observe that it is incorrectly flagged as a broken link.

Proposed resolution

Enhance the internal route detection logic to account for Drupal installations in subfolders.
This should parse the base_path() of the site and prepend it to internal paths before comparison.

Remaining tasks

• Detect the subfolder base path using Drupal's request context or settings.
• Normalize internal paths before validating them.
• Add functional and kernel tests to cover subfolder installations.

User interface changes

None.

API changes

None.

Data model changes

None.