The handling of slashes was broken around listing workspaces in
workspace_key_prefix. While it worked in most places by splitting an
extra time around the spurious slashes, it failed in the case that the
prefix ended with a slash of its own.
A test was temporarily added to verify that the backend works with the
unusual keys, but rather than risking silent breakage around prefixes
with trailing slashes, we also add validation to prevent users from
entering keys with trailing slashes at all.
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