fix(tui): close selection overlays and report rejected handlers (#124876)

openSelector's onSelect fired the async handler with void and only
closed the overlay after a successful await. A rejecting handler (e.g.
setAgent -> setSession against a failing gateway) left the selector
stranded open with an unhandled rejection: the TUI froze on the picker
with no visible cause — a silent dead-end.

Root cause: failure path missing from the overlay lifecycle. The
handler now catches, surfaces the cause via chatLog, and always closes
the overlay.

Regression: /agent selection with a rejecting setSession asserts the
overlay closes and the cause reaches the chat log — fails pre-fix.

src/tui/tui-command-handlers.test.ts

@@ -741,6 +741,28 @@ describe("tui command handlers", () => {
     expect(closeOverlay).toHaveBeenCalledWith(overlayHandle);
   });
 
+  it("closes the overlay and reports the cause when a selection handler rejects", async () => {
+    const setSession = vi
+      .fn()
+      .mockRejectedValue(new Error("gateway unavailable")) as SetSessionMock;
+    const { handleCommand, openOverlay, closeOverlay, overlayHandle, addSystem } = createHarness({
+      setSession,
+      agents: [{ id: "work" }],
+    });
+
+    await handleCommand("/agent");
+    const selector = firstMockArg(openOverlay, "openOverlay") as SelectableOverlay;
+    selector?.onSelect?.({ value: "work", label: "work" });
+    await flushAsyncSelect();
+
+    // The selector must not stay stranded open on a rejected selection, and
+    // the failure must reach the chat log instead of an unhandled rejection.
+    expect(closeOverlay).toHaveBeenCalledWith(overlayHandle);
+    expect(
+      addSystem.mock.calls.some(([line]) => String(line).includes("gateway unavailable")),
+    ).toBe(true);
+  });
+
   it("forwards /context list directly", async () => {
     const { handleCommand, sendChat, openOverlay } = createHarness();
 

src/tui/tui-command-handlers.ts

@@ -304,7 +304,13 @@ export function createCommandHandlers(context: CommandHandlerContext) {
   ) => {
     selector.onSelect = (item) => {
       void (async () => {
-        await onSelect(item.value);
+        try {
+          await onSelect(item.value);
+        } catch (err) {
+          // A rejected selection must not strand the overlay open with an
+          // unhandled rejection; close it and surface the cause in chat.
+          chatLog.addSystem(`selection failed: ${formatTuiErrorMessage(err)}`);
+        }
         closeOverlayAndRender(overlayHandle);
       })();
     };