Change OAK tests to use BATS

What does this MR do?

This change refactors the testing approach for OpenBao (a security vault service) integration with GitLab's web server configuration. Instead of using a single bash script with manual test assertions, the code now uses BATS (a proper testing framework) to run structured, automated tests.

The tests verify two scenarios: first, that when OpenBao is disabled, no proxy configuration exists and requests are properly rejected; second, that when OpenBao is enabled through GitLab's configuration, the web server correctly creates proxy rules to route OpenBao requests to the right backend service.

The main improvements include better test organization, cleaner setup/teardown of test environments, and more reliable test execution. The functionality being tested remains the same - ensuring GitLab properly configures its web server to act as a reverse proxy for OpenBao when the feature is enabled.

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• Latest Merge Result pipeline is green.
• When ready for review, MR is labeled workflowready for review per the Distribution MR workflow.

For GitLab team members

If you don't have access to this, the reviewer should trigger these jobs for you during the review process.

• The manual Trigger:ee-package jobs have a green pipeline running against latest commit.
• If config/software or config/patches directories are changed, make sure the build-package-on-all-os job within the Trigger:ee-package downstream pipeline succeeded.
• If you are changing anything SSL related, then the Trigger:package:fips manual job within the Trigger:ee-package downstream pipeline must succeed.
• If CI configuration is changed, the branch must be pushed to dev.gitlab.org to confirm regular branch builds aren't broken.

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes.
• Documentation created/updated.
• Tests added.
• Integration tests added to GitLab QA.
• Equivalent MR/issue for the GitLab Chart opened.
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.