Bug: tool_calls messages omit content field

[qznc]

When using chat_with_tools, assistant messages that include tool_calls are serialized without a content field. OpenRouter (and potentially other strict OpenAI-compatible APIs) reject these with:

Every message in the conversation, including those with tool_calls, must include a content field, even if it's an empty string ("").

Root cause

In src/providers/openai_compatible.rs, chat_message_to_openai_message sets content: None for MessageType::ToolUse:

content: match &chat_msg.message_type {
    MessageType::Text => Some(Right(chat_msg.content.clone())),
    // ...
    MessageType::ToolUse(_) => None,   // <-- content is dropped entirely
    MessageType::ToolResult(_) => None,
},

Since OpenAIChatMessage.content is annotated with #[serde(skip_serializing_if = "Option::is_none")], the field is completely absent from the serialized JSON. The ChatMessage may actually carry content (e.g. a reasoning string alongside the tool call), but it is silently discarded.

Expected behavior

The content field should always be present. For ToolUse messages it should be set to chat_msg.content (which may be an empty string):

MessageType::ToolUse(_) => Some(Right(chat_msg.content.clone())),

Observed in llm 1.3.7.