[Issue] improve and make more strict autoload.php

[m2-assistant]

This issue is automatically created based on existing pull request: #28923: improve and make more strict autoload.php

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Preconditions (*)

two improvements:

1. use is_readable instead of file_exists. if we use require or include on a existing file that is not readable php generate errors. So we need to chech if file_exists and is_readable (is_readable do both things)
2. provide an anonymous function to determinate $vendorAutoload, it's allow us to have a more readable code with early return.

Related Pull Requests

none

Fixed Issues (if relevant)

none

Steps to reproduce(*)

not necessary, if CI passes this code is good, it's only technical improvement.

Actual Result:(*)

Expected Result:(*)

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)
• All automated tests passed successfully (all builds are green)

[magento-engcom-team]

✅ Confirmed by @engcom-Alfa
Thank you for verifying the issue. Based on the provided information internal tickets MC-36742 were created

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