feat(llm): 替换 ToolChoice 的自动派生为手动序列化

处理自定义 JSON 序列化逻辑，支持字符串值（`none`/`auto`/`required`）和对象格式（`{"type":"function","function":{"name":"..."}}`）。反序列化时向前兼容两种格式。

src/llm/types/request.rs

@@ -11,8 +11,7 @@ pub struct StreamOptions {
     pub include_obfuscation: Option<bool>,
 }
 
-#[derive(Debug, Clone, Serialize, Deserialize)]
-#[serde(rename_all = "snake_case", tag = "type")]
+#[derive(Debug, Clone)]
 pub enum ToolChoice {
     None,
     Auto,
@@ -21,11 +20,71 @@ pub enum ToolChoice {
         name: String,
     },
     AllowedTools {
-        #[serde(rename = "tools")]
         tool_names: Vec<String>,
     },
 }
 
+impl Serialize for ToolChoice {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        match self {
+            ToolChoice::None => serializer.serialize_str("none"),
+            ToolChoice::Auto => serializer.serialize_str("auto"),
+            ToolChoice::Required => serializer.serialize_str("required"),
+            ToolChoice::Named { name } => {
+                let obj = serde_json::json!({
+                    "type": "function",
+                    "function": { "name": name }
+                });
+                obj.serialize(serializer)
+            }
+            ToolChoice::AllowedTools { tool_names } => {
+                let obj = serde_json::json!({
+                    "type": "function",
+                    "function": { "name": tool_names.first().cloned().unwrap_or_default() }
+                });
+                obj.serialize(serializer)
+            }
+        }
+    }
+}
+
+impl<'de> Deserialize<'de> for ToolChoice {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        let value = Value::deserialize(deserializer)?;
+        match value {
+            Value::String(s) => match s.as_str() {
+                "none" => Ok(ToolChoice::None),
+                "auto" => Ok(ToolChoice::Auto),
+                "required" => Ok(ToolChoice::Required),
+                _ => Err(serde::de::Error::custom(format!("unknown tool choice: {s}"))),
+            },
+            Value::Object(obj) => {
+                let typ = obj.get("type").and_then(|v| v.as_str()).ok_or_else(|| {
+                    serde::de::Error::custom("missing 'type' field in tool_choice")
+                })?;
+                if typ == "function" {
+                    let func = obj.get("function").and_then(|v| v.as_object()).ok_or_else(|| {
+                        serde::de::Error::custom("missing 'function' field in tool_choice")
+                    })?;
+                    let name = func.get("name").and_then(|v| v.as_str()).ok_or_else(|| {
+                        serde::de::Error::custom("missing 'function.name' in tool_choice")
+                    })?;
+                    Ok(ToolChoice::Named { name: name.to_string() })
+                } else {
+                    Err(serde::de::Error::custom(format!("unknown tool_choice type: {typ}")))
+                }
+            }
+            _ => Err(serde::de::Error::custom("tool_choice must be a string or object")),
+        }
+    }
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 #[serde(rename_all = "snake_case", tag = "type")]
 pub enum OpenaiTool {