Optional request extension fields had no size bound. A very large field
could exceed the runtime's string limit while the request was serialized,
which failed every attempt, including retries.
- Bound the extension payload carried by one request (8 MiB by default,
configurable) and send the remainder with later requests.
- If the combined request still cannot be serialized, send the base
request without extension fields and log which fields were omitted.
- Add regression tests and update the related references and decision note.
Yichen Jiang
193f9ce413