Dialogs, file pickers, and closing by the program's leave

(bjogui dialogs) has alert, confirm and prompt, the open, save and folder
pickers, and quit, each a command whose answer comes back as a message. A
dialog is a window, which only the UI thread may make, so the command posts
it there and awaits its Task in a fiber. Avalonia's dialogs take no
CancellationToken, so they cannot be task->event; the Task is awaited
through the non-generic Task.WhenAll, and its result read by reflection,
because Bjolang reads no Task's Result.

With no window, as under start-app, each answers as if cancelled.

run-app's #:on-close turns the close button into a message, and quit
closes the windows by program, which is not asked again.

🤖 Generated with [ECA](https://eca.dev) (anthropic/claude-opus-4-8)

Co-Authored-By: eca-agent <git@eca.dev>

tests/dialogs.bjo

@@ -0,0 +1,54 @@
+;; This Source Code Form is subject to the terms of the Mozilla Public
+;; License, v. 2.0. If a copy of the MPL was not distributed with this
+;; file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+;; bjogui/tests/dialogs.bjo — dialogs and pickers with no window.
+;;
+;;     bjo run tests/dialogs.bjo
+;;
+;; A dialog needs a window to belong to, and an app started with `start-app`
+;; has none. So each answers as if it was cancelled, and that is what this
+;; checks: that the command goes to the UI thread and back, and its message
+;; comes. A dialog on a screen is looked at by a person.
+
+(import (std prelude))
+(import (std simpletest))
+(import (bjogui view))
+(import (bjogui app))
+(import (bjogui dialogs))
+
+(import/extern
+  (sleep-ms (: System.Threading.Thread.Sleep (-> int void) #:blocking)))
+
+(type (: Msg (Union Ask (: Answer string))))
+
+(: (-> Msg (List string) (Tuple (List string) (List (Event Msg)))))
+(defun (gui-update msg answers)
+  (match msg
+    (Ask (next answers
+               (alert "Saved." (Answer "alert"))
+               (confirm "Delete?" (fun (b) (Answer #"confirm ${b}")))
+               (prompt "Name?" (fun (s) (Answer #"prompt ${(option-ref-or s "none")}")) #:value "Ada")
+               (open-file (fun (p) (Answer #"open-file ${(option-ref-or p "none")}")))
+               (open-files (fun (ps) (Answer #"open-files ${(length ps)}")))
+               (save-file (fun (p) (Answer #"save-file ${(option-ref-or p "none")}")) #:name "a.txt")
+               (open-folder (fun (p) (Answer #"open-folder ${(option-ref-or p "none")}")))
+               (quit)))
+    ((Answer s) (next (cons s answers)))))
+
+(defun (main)
+  (def app (start-app '() gui-update (fun (m) (text "")) (fun (c) unit)))
+  (app-dispatch! app Ask)
+  (let go ((tries 0))
+    (app-run-pending! app)
+    (when (and (< (length (app-model app)) 7) (< tries 300))
+      (sleep-ms 10)
+      (go (+ tries 1))))
+  (def answers (list-sort (app-model app)))
+  (expect "every dialog answered" (length answers) 7)
+  (expect "the answers with no window"
+          answers
+          (list-sort '("alert" "confirm #f" "prompt none" "open-file none" "open-files 0"
+                       "save-file none" "open-folder none")))
+  (app-stop! app)
+  (println "bjogui dialogs ok"))