Address Copilot PR review: reject unknown archive formats, fix case-sensitive check

- Add explanatory comment to empty except KeyError block in _extract_workflow_yml
- Use case-insensitive extension matching for local archive detection in workflow add
- Reject unknown archive formats with clear error messages instead of silently
  defaulting to ZIP in preset add --from, extension add --from, download_extension(),
  and download_pack()

src/specify_cli/extensions.py

@@ -2151,6 +2151,11 @@ class ExtensionCatalog:
             raise ExtensionError(f"Failed to save extension archive: {e}")
 
         # Choose file extension based on detected format.
+        if not archive_fmt:
+            raise ExtensionError(
+                f"Could not determine archive format for {download_url}. "
+                "Ensure the URL points to a .zip or .tar.gz file."
+            )
         if archive_fmt == "tar.gz":
             archive_filename = f"{extension_id}-{version}.tar.gz"
         else: