Fix IndexError on empty tool_calls list in proxy convert

[hobostay]

Summary

• tool_calls_to_openai() and tool_calls_to_sse_events() both access tool_calls[0] without checking if the list is empty
• If an upstream filtering step ever removes all tool calls, this raises an unhandled IndexError
• Add early-return guards that fall back to the corresponding text response helpers when the list is empty

Impact

• Without this fix, an empty tool_calls list would crash the proxy's response serialization with a 500 error instead of gracefully returning an empty response

Test plan

• Call tool_calls_to_openai([]) and verify it returns a valid response without IndexError
• Call tool_calls_to_sse_events([]) and verify it returns valid SSE events without IndexError
• Verify normal tool call conversion still works

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