One round for a chat, as many as it takes for an agent

Two different jobs were sharing one number. A plain conversation asking a
question is one round of looking things up and then an answer; the rounds after
that were a small model that had decided searching was the answer searching
until the context ran out, at a full request each. MAX_ROUNDS is 1 now. Several
tools can still be called within that round, which is the thing worth telling
the model.

The trade is real and worth naming: a plain chat can no longer search and then
read one of the results, because reading is a second round. That is what an
agent chat is for.

An agent chat is sized by Limits instead, where steps is now a runaway backstop
and not a working budget. It was 40 and it was reached -- a step count low
enough to be the thing that ends a reply is a count that ends it halfway. What
bounds one now is the wall clock and a new completion-token ceiling, with zero
meaning no ceiling, the same convention index_chars already uses.

That ceiling would have been decorative. generation.completion_tokens is only
populated when the endpoint sends a usage block, and llama.cpp, Ollama and
friends never do; the fallback estimate is computed once, in _run's finally,
long after the loop that needs it. So _written takes the larger of reported and
estimated, and there is a test that runs the whole thing against a stream
reporting no usage at all. A limit that works on OpenAI and silently does
nothing everywhere else is the worst kind: one that looks configured.

core.rounds could not stay one fragment. "You get at most N rounds" is not the
same sentence with a different number in it -- a model told it has a budget
rations it and stops early to report progress, which is exactly the behaviour
that strands a long piece of work. So it splits: core.rounds keeps the
one-round case and gates on a new round_budget variable that _agent_values
blanks, and core.keep_working says the other thing to an agent chat.

A queued message during a one-round reply is now never taken mid-reply -- there
is no work under way to steer -- and falls through to _drain, which gives it a
reply of its own. No code change went with that; it falls out of the guard, and
there is a test so that "it happens to work" and "it is meant to work" stop
looking the same.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/services/agent/session.py

@@ -141,9 +141,12 @@ def resolve(db: DBSession, chat: Chat, user: User | None) -> AgentContext | None
         allow=tuple(values.get("allow_default") or ()),
         deny=tuple(values.get("deny_default") or ()),
         limits=Limits(
-            steps=int(values.get("max_steps") or 40),
+            steps=int(values.get("max_steps") or 200),
             wall_seconds=float(values.get("max_wall_seconds") or 900),
             output_bytes=int(values.get("max_total_output_bytes") or 1024 * 1024),
+            # `or 0` would turn a deliberate 0 into the default, and 0 is how an
+            # administrator says "no ceiling". `agents()` has already clamped it.
+            completion_tokens=int(values.get("max_completion_tokens", 200_000) or 0),
         ),
         timeout=float(values.get("default_timeout") or 60),
         max_timeout=float(values.get("max_timeout") or 600),