fix(formatter): set content to None when tool_calls exists in DeepSeekChatFormatter

[yuyuyu-alan]

Summary

• Fix DeepSeekChatFormatter to set content to None when tool_calls exists
• DeepSeek API doesn't allow assistant message to have both content and tool_calls
• When both exist, DeepSeek interprets it as "interrupted speech" and may repeat the content in the next iteration

Test Plan

• Added test case test_chat_formatter_with_text_and_tool_use to cover the scenario where a message contains both TextBlock and ToolUseBlock
• All existing tests pass

🤖 Generated with Claude Code

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