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/bootstrap.py

@@ -149,11 +149,11 @@ class LogBuffer:
 
         return 0 if self._truncated else self._remaining
 
-    def push(self, text: str) -> None:
+    def push(self, text: str, open: bool = False) -> None:
         with self._lock:
-            self._push_locked(text)
+            self._push_locked(text, open)
 
-    def _push_locked(self, text: str) -> None:
+    def _push_locked(self, text: str, open: bool = False) -> None:
         if self._truncated:
             return
         # Cheap lower bound FIRST: one char is at least one UTF-8 byte and the
@@ -193,7 +193,7 @@ class LogBuffer:
             self._sink(log_truncation_marker(self._max_bytes), truncated=True)
             return
         self._remaining -= cost
-        self._sink(text)
+        self._sink(text, open=open)
 
 
 class _LogStream(io.TextIOBase):
@@ -445,7 +445,10 @@ class _LogStream(io.TextIOBase):
                 self._pending = []
                 self._pending_blocks = []
                 self._pending_chars = 0
-                self._logs.push(line)
+                # The line has NO trailing newline: mark the frame `open` so the
+                # host appends the next log frame to the same entry instead of
+                # inserting a fake newline between two entries.
+                self._logs.push(line, open=True)
 
 
 # ---------------------------------------------------------------------------
@@ -1044,9 +1047,14 @@ async def _run(channel: ProtocolChannel) -> None:
         # The sink writes through the def-time bound encode+write primitives
         # (not _send_sync_cls, whose body still resolves _encode_json_plain and
         # self.write_encoded at call time) so a rebind cannot break a log frame.
-        sink=lambda text, truncated=False: _write_encoded_cls(
+        sink=lambda text, truncated=False, open=False: _write_encoded_cls(
             _encode_plain_cls(
-                {"type": "log", "text": text, **({"truncated": True} if truncated else {})}
+                {
+                    "type": "log",
+                    "text": text,
+                    **({"truncated": True} if truncated else {}),
+                    **({"open": True} if open else {}),
+                }
             )
         ),
     )