fix: serialize Claude tool-use round trips with Anthropic message blocks #3056

Claude tool-use round trips need Anthropic-native message blocks

ReActAgent appends an assistant message containing tool_calls and one or more tool-result messages before the next provider request. ClaudeProvider.generate() currently serializes those messages with the generic OpenAI-shaped Message.to_dict() payload (tool_calls on the assistant and role: tool for results). Anthropic Messages API expects tool_use blocks on the assistant turn and tool_result blocks inside a user turn.

This issue tracks a focused fix to serialize the existing provider-neutral messages into the Anthropic wire format and cover the round trip with unit tests.