Make render_agent_with_queue derive the queue credential path from
agent_client_path() rather than re-interpolating Kind::Hive and the
queue/agent segment. This enforces path agreement through code reuse
instead of two hand-written spellings that can diverge.
The policy stanza needs the policy form (secret/data/...) while
agent_client_path returns the client form (swarm/hives/...). Bridge
this by wrapping agent_client_path's result with {MOUNT}/data/, the
same mechanism render_agent uses for agent paths.
Update tests to derive expected paths from agent_client_path instead
of hard-coding them, so a future divergence fails the test rather
than passing silently.
Refs #4386
f4b7a1e357