[Feature] task: route all task sessions through single constructor

[Feature] task: route all task sessions through single constructor

Only worker.c and 1 of 8 controller.c task sessions had the item-name
resolver wired, so timeout logs for scans from other entry points
(check, learn, scan-lua, worker_util startup, proxy, lua.task_from_mime)
lost symbol context entirely: the summary said only "rspamd lua http=1"
with no hint of which rule stalled.

Introduce rspamd_task_create_session() that wraps session_create and
set_item_name_resolver, and switch all 11 task-scoped session creation
sites to use it. The two controller sites with cbdata user_data (stats,
metrics) stay on rspamd_session_create — they do not execute symcache.

While here, collapse rspamd_session_describe_pending into a single
line that folds symbol/label info into each group key, e.g.
"total=5; rspamd dns[RBL_FOO]=3, rspamd lua http[X]=1". Previously the
symbol detail was a separate line that was silently omitted whenever
the resolver was not wired — exactly the breakage this fixes.