Refuse what a node cannot publish, and stop timing out work that is fine

Four things the python SDK turned up, each fixed where every client sees it.

A key no port declares is now an error rather than a silent drop, on the
return, the yield and the emit alike — the contract the docs already stated.
The SDK reads literal yields at sync time, so a typo fails before anything
runs, and an emission of one fails the call rather than being logged where
nobody looks.

NaN and infinity are refused at the port. JSON cannot spell either, so one
that travelled came back as a 500, a socket frame that stopped the canvas, or
a metric batch the database dropped whole.

An artifact input takes `@run:<id>.<output>` or a bare digest, resolved on the
engine — so the CLI, the run dialog and a python caller mean the same thing,
and a sweep can pass one at all.

Node timeouts are off by default. The clock measured silence, which a training
node is full of, and remote workers had already stopped enforcing it — their
heartbeat reset it. Now a heartbeat proves the agent rather than the node,
ninety seconds of nothing fails the call either way, and the engine touches
work it is still running so a long node is not redelivered at sixty seconds.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019V5bsYGNxcgPs4xXmTPx69

backend/tests/flow/test_runs.py

@@ -8,6 +8,7 @@ parameters a caller sends are refused before anything runs if they are wrong.
 
 import pytest
 
+from fluksio.flow.events import EventBus
 from fluksio.flow.messages import DType, MessageSpec
 from fluksio.flow.nodes import Node
 from fluksio.flow.pipeline import NodeOutcome, Pipeline, run_cache_key
@@ -241,17 +242,24 @@ def test_emissions_are_checked_against_the_port_they_name():
     assert "loss" in seen[0].error
 
 
-def test_an_emission_that_nothing_declares_is_ignored():
+def test_an_emission_that_nothing_declares_names_what_was_emitted():
+    """A mistyped metric name is how a training curve goes missing."""
+    events = []
+
     def stray(params):
         yield {"undeclared": 1.0}
         return {"final_loss": 2.0}
 
+    bus = EventBus()
+    bus.publish = events.append  # type: ignore[method-assign]
     node = make_node("train", "study", stray, provides=[spec("final_loss")])
     state = MemoryState()
-    Pipeline(nodes=[node], state=state).run()
+    Pipeline(nodes=[node], state=state, events=bus).run()
 
+    (error,) = [e for e in events if e["type"] == "node_error"]
+    assert "undeclared" in error["error"]
+    assert "final_loss" in error["error"]
     assert "study.undeclared" not in state
-    assert state["study.final_loss"] == 2.0
 
 
 def test_emissions_reach_the_run_as_a_series_with_a_step_each():
@@ -435,3 +443,4 @@ def test_a_node_with_no_fingerprint_is_never_looked_up():
     assert calls == [1]
     assert cache.asked == []
     assert seen[0].cache_key == ""
+