fix(gemini): resolve compilation errors and enable Gemini 3 reasoning

src/models/mod.rs

@@ -223,6 +223,7 @@ pub struct UnifiedRequest {
 pub struct UnifiedMessage {
     pub role: String,
     pub content: Vec<ContentPart>,
+    pub reasoning_content: Option<String>,
     pub tool_calls: Option<Vec<ToolCall>>,
     pub name: Option<String>,
     pub tool_call_id: Option<String>,
@@ -337,6 +338,7 @@ impl TryFrom<ChatCompletionRequest> for UnifiedRequest {
                 UnifiedMessage {
                     role: msg.role,
                     content,
+                    reasoning_content: msg.reasoning_content,
                     tool_calls: msg.tool_calls,
                     name: msg.name,
                     tool_call_id: msg.tool_call_id,