fix(openai): preserve assistant content when converting Responses tool-call turns

- Merge buffered `tool_calls` into the latest mergeable assistant message instead of always appending a new one.
- Combine deferred reasoning segments across the same assistant turn, while ignoring `[reasoning unavailable]` placeholders.
- Reset merge state on role/tool-output boundaries to keep tool-call attachment behavior correct across turns.

Closes: #4676