Introduce thread safety to SAML schema read

[e-ikuta]

Problem

As the related PR below, use of Dir.chdir is not thread-safe. This usage was causing exceptions to be raised when running on Puma and in other multi-threaded environments.

Related:

• Introduce thread safety to SAML schema validation #175

How to fix

This patch replaces Dir.chdir with thread-safe File.open method.

Additional information

This patch also removes @@mutex, which became unnecessary.

[alpaca-tc]

@johnnyshields @pitbulk If everything looks good to you, I would greatly appreciate it if you could consider merging this PR 😄

In the service I manage, we occasionally encounter a rare issue where file reading fails with an error in production. After investigating, we identified that the root cause is a Dir.chdir call in another thread, which changes the current working directory unexpectedly.

This PR would resolve the issue and ensure stable file reading.

[johnnyshields]

@pitbulk lgtm, please merge.