Fix Gemini 3 on OpenRouter (#43416)

Release Notes:

- Gemini 3 now works on OpenRouter in the Agent Panel

crates/agent_ui/src/buffer_codegen.rs

@@ -423,6 +423,7 @@ impl CodegenAlternative {
                 role: Role::User,
                 content: Vec::new(),
                 cache: false,
+                reasoning_details: None,
             };
 
             if let Some(context) = context_task.await {

crates/agent_ui/src/terminal_inline_assistant.rs

@@ -262,6 +262,7 @@ impl TerminalInlineAssistant {
                 role: Role::User,
                 content: vec![],
                 cache: false,
+                reasoning_details: None,
             };
 
             if let Some(context) = load_context_task.await {