Backport of UI: Correctly call resultant-acl endpoint when user root is root into release/1.14.x

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

Backport

This PR is auto-generated from #25766 to be assessed for backporting due to the inclusion of the label backport/1.14.x.

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

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In #25256 we updated the permissions adapter logic to call the resultant-acl endpoint with the user's namespace, if it exists. However, the logic did not work for users in the root namespace who log into a child namespace. Now it does, with test coverage.

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Overview of commits

• cdd88d5

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CI Results:
All Go tests succeeded! ✅

[hashishaw]

original change not backported to 1.14, closing out