Doorkeeper: Support HTTP in discovery requests

What does this MR do and why?

We found in #554589 (closed) that when running GitLab over HTTP, OIDC discovery will fail because:

1. Client requests the /configuration endpoint over HTTP. This works.
2. GitLab responds with an HTTPS jwks_uri back to itself.
3. Client requests an HTTPS URL - this will fail.

We should ensure that if clients request HTTP, they should get HTTP URLs in response.

References

• #554589 (closed)
• https://github.com/doorkeeper-gem/doorkeeper-openid_connect?tab=readme-ov-file#configuration

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Run AI gateway with AIGW_GITLAB_URL=http://<your_instance>
2. Send request e.g. curl -v -H'Authorization: Bearer jwt' -H'x-gitlab-authentication-type: oidc' http://localhost:5001/v4/code/completion (token doesn't matter since it never gets to validate it anyway)
3. AIGW will 500 when trying to resolve /oauth/discovery/keys

With this MR applied, step 3 should succeed instead.

MR acceptance checklist

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Related to #554589 (closed)