test(copilot): fix loadWorkspaceFileTextForPreview mock to return { text } not a bare string

The adapter reads previewBase.text to seed an append/patch base; the mock returned
a bare '' so previewBase.text was undefined, making a base-less append fail closed
(no file_preview_content). My PR's fail-close change exposed the wrong-shaped mock.

Author: waleedlatif1

apps/sim/lib/copilot/request/go/stream.test.ts

@@ -34,7 +34,10 @@ vi.mock('@/lib/copilot/tools/server/files/file-preview', async () => {
   >('@/lib/copilot/tools/server/files/file-preview')
   return {
     ...actual,
-    loadWorkspaceFileTextForPreview: vi.fn().mockResolvedValue(''),
+    // Returns the file's preview base as a `WorkspaceFilePreviewBase` ({ text }), NOT a bare string —
+    // the adapter reads `previewBase.text` to seed an append/patch base. An empty base ('') is defined,
+    // so a base-less append doesn't fail closed.
+    loadWorkspaceFileTextForPreview: vi.fn().mockResolvedValue({ text: '' }),
   }
 })