## Why
History estimation and executed-tool-call limits only need the size of serialized JSON, not the serialized output itself.
## What changed
- Add a shared `serialized_json_bytes` helper that streams JSON into a byte counter.
- Use it for response-item size estimates and executed-tool-call argument limits.
- Preserve serialization errors so callers can apply their existing fallback behavior.
## Testing
- Cover response-item sizing with escaped characters, newlines, and multibyte text.
GitOrigin-RevId: a4417996df96df5c51cae62830adc496912f69c7