fix(code-runtime-python): merge a flushed unterminated line into the next log entry

The review's remaining warning: an explicit flush of an unterminated line
(print(..., end='', flush=True)) pushed a full log frame, so the following
print() landed in a second entry and logs.join('\n') rendered 'a\nb' for what
the program printed as one line — a model-visible output defect. The flush
frame now carries an  flag (LogMessage gains the optional field on both
sides and in the mirror test), the host holds it and appends the next log frame
to the same entry, and finish() admits the residual if the run ends with it
still open. The settlement note registers the decimal-context fix from the
previous commit.

packages/code-runtime/code-runtime-python/py/protocol.py

@@ -81,10 +81,11 @@ class LogMessage(_LogMessageRequired, total=False):
 
     ``truncated`` is set only on the frame that IS the child ledger's truncation
     marker (not program output), so the host stops capturing at the same point
-    the child did — mirrors the TS `truncated?`.
+    the child did — mirrors the TS `truncated?`. ``open`` is set on a flushed unterminated line the host appends the next frame to (mirrors `open?`).
     """
 
     truncated: bool
+    open: bool
 
 
 class DoneErrorField(TypedDict):