ecmwf-sites-upload: handle single-file upload result and add recursive option

[Copilot]

upload() returns a str for single-file uploads but the success check only handled list, causing false failures. Additionally, recursive=True was hardcoded with no way to override it.

Changes

• Single-file result handling (upload.py): Extend success condition to accept str in addition to non-empty list:

  # before
  if isinstance(res, list) and len(res):
  # after
  if (isinstance(res, list) and len(res)) or isinstance(res, str):

• --recursive argument (upload.py): Added optional CLI flag defaulting to True; accepts string input ("false" → False, anything else → True):

  parser.add_argument("--recursive", type=lambda x: x.lower() != "false", default=True)

  Used in the upload call: recursive=args.recursive

• Validation loop fix (upload.py): Changed if not v → if v is None to prevent --recursive false from incorrectly triggering a "not provided" error.

• action.yml: Exposed recursive as an optional input (default "true") and passes it through to the script.