fix(runtime): use consistent error handling for user input API

- Change 'not loaded' to 'not found' in submit_user_input and
  cancel_user_input so map_thread_err correctly maps to 404
- Use map_thread_err in submit_user_input API endpoint for
  consistent error response (404 for missing thread, 409 for
  conflict, etc.) instead of always returning 500

crates/tui/src/runtime_api.rs

@@ -1055,7 +1055,7 @@ async fn submit_user_input(
         .runtime_threads
         .submit_user_input(&thread_id, &input_id, response)
         .await
-        .map_err(|e| ApiError::internal(format!("Failed to submit user input: {e}")))?;
+        .map_err(map_thread_err)?;
     Ok(Json(SubmitUserInputResponse {
         ok: true,
         input_id,

crates/tui/src/runtime_threads.rs

@@ -841,7 +841,7 @@ impl RuntimeThreadManager {
     ) -> Result<bool> {
         let active = self.active.lock().await;
         let Some(state) = active.engines.get(thread_id) else {
-            bail!("thread '{thread_id}' not loaded");
+            bail!("thread '{thread_id}' not found");
         };
         state.engine.submit_user_input(input_id, response).await?;
         Ok(true)
@@ -851,7 +851,7 @@ impl RuntimeThreadManager {
     pub async fn cancel_user_input(&self, thread_id: &str, input_id: &str) -> Result<bool> {
         let active = self.active.lock().await;
         let Some(state) = active.engines.get(thread_id) else {
-            bail!("thread '{thread_id}' not loaded");
+            bail!("thread '{thread_id}' not found");
         };
         state.engine.cancel_user_input(input_id).await?;
         Ok(true)