Backport of Fix MCP OAuth discovery failing on relative URL installs

What does this MR do and why?

Backports !226962 (merged)

The MCP OAuth discovery endpoint at /.well-known/oauth-authorization-server/api/v4/mcp returned the wrong scopes_supported value on instances served under a relative URL root (e.g. gitlab.example.com/gitlab). The controller checked request.path, which on a relative-URL install includes the prefix, so it never matched the hardcoded /api/v4/mcp suffix. The fix switches to request.path_info (the path after the relative-root prefix) so the MCP scope filter matches correctly

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
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  • If this MR fixes a bug that affects customers, the customer label has been applied.
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• 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, reach out to a Software Engineer in Test in #s_developer_experience.

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