Function downloadMaterial

downloadMaterial(
    context: ExtensionContext,
    item: BBMaterialItem,
    saveDirectlyToWorkspace?: boolean,
): Promise<void>
Downloads one file from Blackboard into the user’s workspace.

Flow ：
1. Validate the tree item is a file.
2. Ask user “where to save” via vscode.window.showSaveDialog, or to workspace directly.
3. Ensure Blackboard session (CredentialManager + CasClient).
4. Stream the file to disk with progress UI.
5. Notify success / failure.
Parameters
- context: ExtensionContext
  VS Code extension context.
- item: BBMaterialItem
  Blackboard tree item representing a file.
- saveDirectlyToWorkspace: boolean = false
  If true, skips the save dialog.
Returns Promise<void>
- Defined in src/backend/commands/downloadMaterial.ts:30