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/fluksio/core/config.py

@@ -93,10 +93,11 @@ class Settings(BaseSettings):
     MCP_TOKEN_EXPIRE_MINUTES: int = 60
     MCP_REFRESH_EXPIRE_DAYS: int = 30
     FLOW_MAX_WORKERS: int = 4
-    # How long a python node may run before its worker is killed, unless the
-    # node sets its own. Long enough for a slow HTTP call, short enough that a
-    # runaway loop is not a wedged flow.
-    FLOW_NODE_TIMEOUT: float = 30.0
+    # How long a python node may be silent before its worker is killed, unless
+    # the node sets its own. 0, the default, disables it: a dead worker still
+    # fails fast, and a slow one is left to finish. Set it where silence means
+    # stuck rather than working.
+    FLOW_NODE_TIMEOUT: float = 0.0
     # How long the engine's own metrics, events and run records are kept.
     OBS_RETENTION_DAYS: int = 30
     # Without a Redis host the engine keeps its state in memory.