Backport of UI/add managed ns redirect prefix into release/1.9.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #14422 to be assessed for backporting due to the inclusion of the label backport/1.9.x.

WARNING automatic cherry-pick of commits failed. Commits will require human attention.

The below text is copied from the body of the original PR.

---

This PR updates the namespace redirect logic on managed vault clusters.

Previous behavior caused a bug where if a namespace was provided in the URL but did not match the required format for managed clusters, it looked like you were logging into the admin namespace when in reality, the namespace passed to all requests is whatever is in the URL param.

UI Behavior when I try to log in with my token which is valid for the admin namespace:

Fails because request has the namespace which is in the URL param:

After this fix, whatever namespace passed is parsed and either appended to the root (admin) or kept as is, if the namespace param starts with admin/

New Behavior

[hashicorp-cla]

)

Thank you for your submission! We require that all contributors sign our Contributor License Agreement ("CLA") before we can accept the contribution. Read and sign the agreement)

Learn more about why HashiCorp requires a CLA and what the CLA includes

---

temp seems not to be a GitHub user.
You need a GitHub account to be able to sign the CLA. If you already have a GitHub account, please add the email address used for this commit to your account.

_{Have you signed the CLA already but the status is still pending? Recheck it).}

[hashishaw]

Automatic cherry-pick failed, closing this in favor of #14440