Split tui/mod.rs up into multiple files. (#3)

The mod.rs file was too large, split it up.

Reviewed-on: #3
Co-authored-by: Drew Galbraith <drew@tiramisu.one>
Co-committed-by: Drew Galbraith <drew@tiramisu.one>

src/tui/events.rs

@@ -0,0 +1,72 @@
+//! Server event draining: applies incoming [`UIEvent`]s to the TUI state.
+
+use tokio::sync::mpsc;
+use tracing::debug;
+
+use super::AppState;
+use crate::core::types::{Role, UIEvent};
+
+/// Drain all pending [`UIEvent`]s from `event_rx` and apply them to `state`.
+///
+/// This is non-blocking: it processes all currently-available events and returns
+/// immediately when the channel is empty.
+///
+/// | Event              | Effect                                                     |
+/// |--------------------|------------------------------------------------------------|
+/// | `StreamDelta(s)`   | Append `s` to last message if it's `Assistant`; else push new |
+/// | `TurnComplete`     | No structural change; logged at debug level                |
+/// | `Error(msg)`       | Push `(Assistant, "[error] {msg}")`                        |
+pub(super) fn drain_ui_events(event_rx: &mut mpsc::Receiver<UIEvent>, state: &mut AppState) {
+    while let Ok(event) = event_rx.try_recv() {
+        match event {
+            UIEvent::StreamDelta(chunk) => {
+                if let Some((Role::Assistant, content)) = state.messages.last_mut() {
+                    content.push_str(&chunk);
+                } else {
+                    state.messages.push((Role::Assistant, chunk));
+                }
+            }
+            UIEvent::TurnComplete => {
+                debug!("turn complete");
+            }
+            UIEvent::Error(msg) => {
+                state
+                    .messages
+                    .push((Role::Assistant, format!("[error] {msg}")));
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[tokio::test]
+    async fn drain_appends_to_existing_assistant() {
+        let (tx, mut rx) = tokio::sync::mpsc::channel(8);
+        let mut state = AppState::new();
+        state.messages.push((Role::Assistant, "hello".to_string()));
+        tx.send(UIEvent::StreamDelta(" world".to_string()))
+            .await
+            .unwrap();
+        drop(tx);
+        drain_ui_events(&mut rx, &mut state);
+        assert_eq!(state.messages.last().unwrap().1, "hello world");
+    }
+
+    #[tokio::test]
+    async fn drain_creates_assistant_on_user_last() {
+        let (tx, mut rx) = tokio::sync::mpsc::channel(8);
+        let mut state = AppState::new();
+        state.messages.push((Role::User, "hi".to_string()));
+        tx.send(UIEvent::StreamDelta("hello".to_string()))
+            .await
+            .unwrap();
+        drop(tx);
+        drain_ui_events(&mut rx, &mut state);
+        assert_eq!(state.messages.len(), 2);
+        assert_eq!(state.messages[1].0, Role::Assistant);
+        assert_eq!(state.messages[1].1, "hello");
+    }
+}