Fix Zoekt global code search- Backport

What does this MR do and why?

Backport for Fix Zoekt global code search (!165835 - merged)

Fix Zoekt global code search

There is no Code tab in search navigation now. There are no results with manual scope=blobs now.

This fix:

• uses :zoekt_cross_namespace_search for navigation,
• correctly overrides allowed scopes,
• adds and updates tests.

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).
• This MR has a severity label assigned (if applicable).
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

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).