Tool-call completions carry content=None (the key exists, so a .get
default never applies). Both chunked request paths concatenated it
directly ("" += None -> TypeError), recording a provider failure on
every attempt even though upstream returned 200 - rotation 'lisa'
exhausted codex_think and returned 429 despite available usage.

Coalesce null content to '' and pass tool_calls through to the combined
response with finish_reason "tool_calls" (previously they were silently
discarded and agents received an empty completion).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>