gpui: Support prompt_for_paths in TestPlatform (#58139)

Implements the previously-`unimplemented!()`
`TestPlatform::prompt_for_paths` so tests can drive the platform Open
dialog deterministically.

Adds `TestAppContext::simulate_path_prompt_response` and
`did_prompt_for_paths`, mirroring the existing `prompt_for_new_path`
test helpers (`simulate_new_path_selection`). The simulated response
validates that callers don't return multiple paths when
`PathPromptOptions::multiple` is false.

Release Notes:

- N/A

crates/gpui/src/app/test_context.rs

@@ -336,6 +336,20 @@ impl TestAppContext {
         self.test_platform.simulate_new_path_selection(select_path);
     }
 
+    /// Simulates responding to a `prompt_for_paths` ("Open") dialog.
+    pub fn simulate_path_prompt_response(
+        &self,
+        select_paths: impl FnOnce(&crate::PathPromptOptions) -> Option<Vec<std::path::PathBuf>>,
+    ) {
+        self.test_platform
+            .simulate_path_prompt_response(select_paths);
+    }
+
+    /// Returns true if there's a path selection dialog pending.
+    pub fn did_prompt_for_paths(&self) -> bool {
+        self.test_platform.did_prompt_for_paths()
+    }
+
     /// Simulates clicking a button in an platform-level alert dialog.
     #[track_caller]
     pub fn simulate_prompt_answer(&self, button: &str) {
@@ -1098,3 +1112,54 @@ impl AnyWindowHandle {
             .unwrap()
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::{PathPromptOptions, TestAppContext};
+    use std::path::PathBuf;
+
+    #[gpui::test]
+    async fn test_simulate_path_prompt_response(cx: &mut TestAppContext) {
+        assert!(!cx.did_prompt_for_paths());
+
+        let receiver = cx.update(|cx| {
+            cx.prompt_for_paths(PathPromptOptions {
+                files: false,
+                directories: true,
+                multiple: true,
+                prompt: None,
+            })
+        });
+        assert!(cx.did_prompt_for_paths());
+
+        let selected = vec![PathBuf::from("/a"), PathBuf::from("/b")];
+        cx.simulate_path_prompt_response({
+            let selected = selected.clone();
+            move |options| {
+                assert!(options.multiple);
+                Some(selected)
+            }
+        });
+        assert!(!cx.did_prompt_for_paths());
+
+        let response = receiver.await.unwrap().unwrap();
+        assert_eq!(response, Some(selected));
+    }
+
+    #[gpui::test]
+    async fn test_simulate_path_prompt_cancellation(cx: &mut TestAppContext) {
+        let receiver = cx.update(|cx| {
+            cx.prompt_for_paths(PathPromptOptions {
+                files: true,
+                directories: false,
+                multiple: false,
+                prompt: None,
+            })
+        });
+
+        cx.simulate_path_prompt_response(|_options| None);
+
+        let response = receiver.await.unwrap().unwrap();
+        assert_eq!(response, None);
+    }
+}

crates/gpui/src/platform/test/platform.rs

@@ -1,9 +1,10 @@
 use crate::{
     AnyWindowHandle, BackgroundExecutor, ClipboardItem, CursorStyle, DevicePixels,
-    DummyKeyboardMapper, ForegroundExecutor, Keymap, NoopTextSystem, Platform, PlatformDisplay,
-    PlatformHeadlessRenderer, PlatformKeyboardLayout, PlatformKeyboardMapper, PlatformTextSystem,
-    PromptButton, ScreenCaptureFrame, ScreenCaptureSource, ScreenCaptureStream, SourceMetadata,
-    Task, TestDisplay, TestWindow, ThermalState, WindowAppearance, WindowParams, size,
+    DummyKeyboardMapper, ForegroundExecutor, Keymap, NoopTextSystem, PathPromptOptions, Platform,
+    PlatformDisplay, PlatformHeadlessRenderer, PlatformKeyboardLayout, PlatformKeyboardMapper,
+    PlatformTextSystem, PromptButton, ScreenCaptureFrame, ScreenCaptureSource, ScreenCaptureStream,
+    SourceMetadata, Task, TestDisplay, TestWindow, ThermalState, WindowAppearance, WindowParams,
+    size,
 };
 use anyhow::Result;
 use collections::VecDeque;
@@ -85,6 +86,10 @@ struct TestPrompt {
 pub(crate) struct TestPrompts {
     multiple_choice: VecDeque<TestPrompt>,
     new_path: VecDeque<(PathBuf, oneshot::Sender<Result<Option<PathBuf>>>)>,
+    paths: VecDeque<(
+        PathPromptOptions,
+        oneshot::Sender<Result<Option<Vec<PathBuf>>>>,
+    )>,
 }
 
 impl TestPlatform {
@@ -147,6 +152,33 @@ impl TestPlatform {
         tx.send(Ok(select_path(&path))).ok();
     }
 
+    pub(crate) fn simulate_path_prompt_response(
+        &self,
+        select_paths: impl FnOnce(&PathPromptOptions) -> Option<Vec<std::path::PathBuf>>,
+    ) {
+        let (options, tx) = self
+            .prompts
+            .borrow_mut()
+            .paths
+            .pop_front()
+            .expect("no pending paths prompt");
+        let selection = select_paths(&options);
+        if let Some(paths) = &selection
+            && !options.multiple
+            && paths.len() > 1
+        {
+            panic!(
+                "selected {} paths for a prompt that does not allow multiple selection",
+                paths.len()
+            );
+        }
+        tx.send(Ok(selection)).ok();
+    }
+
+    pub(crate) fn did_prompt_for_paths(&self) -> bool {
+        !self.prompts.borrow().paths.is_empty()
+    }
+
     #[track_caller]
     pub(crate) fn simulate_prompt_answer(&self, response: &str) {
         let prompt = self
@@ -348,9 +380,11 @@ impl Platform for TestPlatform {
 
     fn prompt_for_paths(
         &self,
-        _options: crate::PathPromptOptions,
+        options: crate::PathPromptOptions,
     ) -> oneshot::Receiver<Result<Option<Vec<std::path::PathBuf>>>> {
-        unimplemented!()
+        let (tx, rx) = oneshot::channel();
+        self.prompts.borrow_mut().paths.push_back((options, tx));
+        rx
     }
 
     fn prompt_for_new_path(