fix(opencode): preserve extra model fields during serialization

Add `extra` field with serde flatten to OpenCodeModel struct to capture
custom fields like cost, modalities, thinking, and variants that were
previously lost during save operations.

src-tauri/src/provider.rs

@@ -596,6 +596,11 @@ pub struct OpenCodeModel {
     /// 模型额外选项（provider 路由等）
     #[serde(skip_serializing_if = "Option::is_none")]
     pub options: Option<HashMap<String, Value>>,
+
+    /// 额外字段（cost、modalities、thinking、variants 等）
+    /// 使用 flatten 捕获所有未明确定义的字段
+    #[serde(flatten, default, skip_serializing_if = "HashMap::is_empty")]
+    pub extra: HashMap<String, Value>,
 }
 
 /// OpenCode 模型限制