SSH Remoting: Add the ability to resolve file paths on the remote host (#18250)

Release Notes:

- N/A

crates/remote_server/src/headless_project.rs

@@ -108,6 +108,7 @@ impl HeadlessProject {
         session.subscribe_to_entity(SSH_PROJECT_ID, &settings_observer);
 
         client.add_request_handler(cx.weak_model(), Self::handle_list_remote_directory);
+        client.add_request_handler(cx.weak_model(), Self::handle_check_file_exists);
 
         client.add_model_request_handler(Self::handle_add_worktree);
         client.add_model_request_handler(Self::handle_open_buffer_by_path);
@@ -298,4 +299,20 @@ impl HeadlessProject {
         }
         Ok(proto::ListRemoteDirectoryResponse { entries })
     }
+
+    pub async fn handle_check_file_exists(
+        this: Model<Self>,
+        envelope: TypedEnvelope<proto::CheckFileExists>,
+        cx: AsyncAppContext,
+    ) -> Result<proto::CheckFileExistsResponse> {
+        let fs = cx.read_model(&this, |this, _| this.fs.clone())?;
+        let expanded = shellexpand::tilde(&envelope.payload.path).to_string();
+
+        let exists = fs.is_file(&PathBuf::from(expanded.clone())).await;
+
+        Ok(proto::CheckFileExistsResponse {
+            exists,
+            path: expanded,
+        })
+    }
 }