Fix issue #1834: Spawn/subagent tool fails with Deepseek Reasoner
due to missing reasoning_content field when using thinking mode.
The subagent was not including reasoning_content and thinking_blocks
in assistant messages with tool calls, causing the Deepseek API to
reject subsequent requests.
- Add reasoning_content to assistant message when subagent makes tool calls
- Add thinking_blocks to assistant message for Anthropic extended thinking
- Add tests to verify both fields are properly passed
Fixes#1834
