fix(code-runtime-python): cap the raw frame length before JSON.parse and bound the log sink

Addresses the review's remaining two items:
- FRAME_PARSE_CAP_BYTES (64 MiB) drops an fd-3 frame whose raw length exceeds
  it BEFORE toString/JSON.parse: the 256 MiB wire ceiling bounds the bytes, not
  the decoded structure, and a compact wide frame near that ceiling could decode
  to far more host memory. A regression test writes a 65 MiB log frame plus a
  normal one and asserts the oversized frame is dropped while the trailing frame
  still lands in logs (fail-before: without the cap the oversized text is parsed
  and admitted, truncating the ledger so the trailing frame is dropped). The
  forged-oversized lower-bound test's frame is reduced to stay under the cap
  while still exercising the truncation path.
- The log sink writes through the def-time bound encode+write primitives (not
  send_sync, whose body resolves _encode_json_plain and self.write_encoded at
  call time), so a rebind cannot break a log frame.

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

@@ -1032,8 +1032,13 @@ async def _run(channel: ProtocolChannel) -> None:
 
     logs = LogBuffer(
         int(boot["maxLogBytes"]),
-        sink=lambda text, truncated=False: _send_sync_cls(
-            {"type": "log", "text": text, **({"truncated": True} if truncated else {})}
+        # 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(
+            _encode_plain_cls(
+                {"type": "log", "text": text, **({"truncated": True} if truncated else {})}
+            )
         ),
     )