Revert StringFilter/StringList aliases; use inline PEP 604 unions

Per code-review feedback: domain-specific type aliases hide what the
annotations actually mean. PEP 604 unions are the pythonic idiom and
show the caller exactly what's accepted at the call site.

- `StringFilter` (172 sites)  -> `str | Iterable[str] | None`
- `StringList`   (8 sites)    -> `list[str] | None`

Runtime behavior is unchanged: `_get_args` still routes filter params
through `_normalize_str_iterable` (accepts str, list, tuple, Series,
ndarray, generator) and `properties` through `_LIST_ONLY_STR_PARAMS`
(wraps stray single-string input into a one-element list).

Author: thodson-usgs