Fix RPC reads missing error cause for structured file system errors

Add workspaceFileReadCompatibilityDetail() to properly map each
WorkspaceFileSystemError tag to a user-visible description at the
projectsReadFile RPC boundary.

The previous code accessed cause.cause on all non-outside-root errors,
but WorkspaceFilePathEscapeError, WorkspacePathNotFileError, and
WorkspaceBinaryFileError have no cause property, producing 'undefined'
in client error messages.

Author: cursoragent

apps/server/src/ws.ts

@@ -170,6 +170,23 @@ function workspaceBrowseCompatibilityDetail(
   }
 }
 
+function workspaceFileReadCompatibilityDetail(
+  error: WorkspaceFileSystem.WorkspaceFileSystemError,
+): string {
+  switch (error._tag) {
+    case "WorkspaceFileSystemOperationError":
+      return legacyPlatformFailureDescription(error.cause);
+    case "WorkspaceFilePathEscapeError":
+      return "Workspace file path resolves outside the project root.";
+    case "WorkspacePathNotFileError":
+      return "Workspace path is not a file.";
+    case "WorkspaceBinaryFileError":
+      return "Binary files cannot be previewed as text.";
+    default:
+      return unexpectedCompatibilityError(error);
+  }
+}
+
 function projectSetupScriptCompatibilityDetail(
   error: ProjectSetupScriptRunner.ProjectSetupScriptRunnerError,
 ): string {
@@ -1288,7 +1305,7 @@ const makeWsRpcLayer = (currentSession: EnvironmentAuth.AuthenticatedSession) =>
               Effect.mapError((cause) => {
                 const message = isWorkspacePathOutsideRootError(cause)
                   ? "Workspace file path must stay within the project root."
-                  : `Failed to read workspace file: ${legacyPlatformFailureDescription(cause.cause)}`;
+                  : `Failed to read workspace file: ${workspaceFileReadCompatibilityDetail(cause)}`;
                 return new ProjectReadFileError({ message, cause });
               }),
             ),