job_queue: test error truncation as the pure fn it is

error_is_truncated submitted a DAG, claimed its head, failed it with a
long string and read the error back out of a snapshot -- four moving
parts to observe one `&str -> String`. The DAG round-trip it depended on
is covered by its own tests either way.

Testing truncate_error directly also reaches the case the round-trip
never could: the cap is a byte length, so a multibyte char straddling it
would panic the slice. That boundary scan is the only non-obvious line
in the function and it had no coverage at all -- the old test used a
repeated ASCII 'x', where byte and char offsets coincide.

Also drops a stale claim from insert_job's doc: it has not recorded a
per-node node_rt since that map was deleted.

hive-c0re/src/job_queue/mod.rs

@@ -148,8 +148,7 @@ fn outcome_of(result: Result<(), String>) -> Outcome {
     }
 }
 
-/// Insert a declared `job` into the shared graph and record its per-node
-/// `node_rt`, returning the inserted ids.
+/// Insert a declared `job` into the shared graph, returning the inserted ids.
 ///
 /// A node that declared no parent hangs under `group_parent` — the DAG
 /// container for a template, the emitting node for a runtime-appended