* fix(llm): preserve null content on OpenAI tool-call turns
OpenAI-compatible providers can return assistant tool-call messages with
content=null. Coercing that to "" before history replay breaks providers
that bind opaque reasoning state to the exact assistant message shape.
Keep null only when the normalized response has tool calls; tool-less
null still becomes "", and content_override stays authoritative.
Fixes#1061
* test(live_llm): cover OpenAI null content tool-call replay
Add a live multi-turn tool replay that asserts provider content=null stays
null through normalize + OpenAIHistoryAdapter and that the continuation
still answers. Mark gpt_4/gpt_5 families as supports_tool_replay.
* docs(llm): note content_override None sentinel semantics
None means no override, not force-null content. Addresses review on #1064.
dea2917fa9