Handle keyboard-invoked activateFromFiles without selectedFiles

[Copilot]

Keyboard shortcuts can invoke protein-viewer.activateFromFiles without the Explorer multi-select payload, causing undefined to flow into file rendering and crash on .map(...). This change makes command argument handling resilient across invocation paths (context menu vs keybinding).

• Command input normalization

  • Added getFileUrisToOpen(fileUri, selectedFiles, activeEditorUri) to resolve files in a strict fallback order:
    1. selectedFiles (Explorer multi-select)
    2. command fileUri
    3. active editor document URI
    4. empty list

• Crash guard at panel boundary

  • Updated ProteinViewerPanel.renderFromFiles(...) to short-circuit on empty input and surface a user-facing error instead of dereferencing undefined.

• Type and call-site hardening

  • Updated file-list parameters to accept readonly/optional URI arrays where appropriate.
  • Removed reliance on assumptions that command args are always present in a fixed shape.

• Focused regression coverage

  • Added tests for URI resolution behavior covering:
    • selected-files precedence
    • single-file command fallback
    • active-editor fallback
    • empty-input result

export function getFileUrisToOpen(
  fileUri: vscode.Uri | undefined,
  selectedFiles: readonly vscode.Uri[] | undefined,
  activeEditorUri: vscode.Uri | undefined
): vscode.Uri[] {
  if (selectedFiles?.length) return [...selectedFiles];
  if (fileUri) return [fileUri];
  if (activeEditorUri) return [activeEditorUri];
  return [];
}