Backport of 'Fix HTTP_V2 config leak breaking unrelated specs'

What does this MR do and why?

Backport of !246960 (merged) to 19-2-stable-ee.

Gitlab::HTTP_V2.configuration is a process-global memoized singleton. Two specs mutate it without restoring it: Keeps::Helpers::GitlabApiHelper#initialize (exercised by spec/keeps/helpers/gitlab_api_helper_spec.rb) empties allowed_internal_uris, and the protocol: 'https' example in spec/initializers/gitlab_http_spec.rb rebuilds it with the https scheme. When either runs before an unrelated spec in the same RSpec process, the GitLab instance's own host is no longer treated as internal, so the always-on public_url validation blocks URLs pointing at it (e.g. spec/models/remote_mirror_spec.rb failed with "Url is blocked: Requests to localhost are not allowed"). This surfaced as a flaky master failure in the no_gitaly_transactions scheduled pipeline.

The fix restores the global Gitlab::HTTP_V2 configuration after each affected example so the leak can no longer poison other specs. This is a spec-only change; the cherry-pick applied cleanly and the backport diff is identical to the original MR.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.