ChatMessage gains toolCalls and an opaque providerData; providers may
yield providerData once per turn. The assistant loop appends one
assistant message per model turn. Anthropic replays its own raw blocks
from providerData (no in-memory map) or maps text + tool_use blocks;
OpenAI maps toolCalls to tool_calls.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
