fix(ogc): raise on an unreadable date bound instead of dropping the filter

[thodson-usgs]

Summary

Follow-up to #440. _format_api_dates returned None when any non-blank element of a date argument matched no supported format, and both callers read None as "no date filter":

• waterdata.get_ratings() sent the STAC search with no datetime and silently returned every rating.
• The OGC getters (Water Data and NGWMN) sent an empty time=, and the service answered HTTP 400 "Invalid datetime format", naming no argument.

#440 fixed one input that took this path (".."). Any other unreadable value ("2024-13-45", "Jan 1 2024", ["2024-01-01", "tomorrow"]) still did.

Changes

• ogc/dates.py: _format_one raises ValueError for a non-blank element it can't parse. The message names the caller's argument (name=) and the value, and gives accepted forms, including None for an open end. A range whose ends are all open still returns None (no filter). Returns/Raises/Notes updated.
• Getter docstrings: the 15 time/begin/end/last_modified example blocks, plus get_ratings time and ngwmn.get_water_level datetime, now show the two-value list form with None for an open end. Until now that form was documented only in the private helper.
• NEWS entry marked Behavior change. Only inputs that already failed are affected: for OGC getters, an HTTP 400 becomes a local ValueError. For get_ratings, a silently unfiltered result becomes an error.

Testing

• New: test_format_api_dates_rejects_an_unreadable_bound (single, unsupported format, "Apr", range start/end), test_get_daily_rejects_an_unreadable_time_before_any_request, and test_get_ratings_rejects_an_unreadable_time_before_any_request. The two getter tests also assert that no request is sent.
• The "Apr" case in test_format_api_dates pinned the old None. I moved it to the rejection test, where it still checks that "Apr" isn't mistaken for a duration.
• Offline suite: 1254 passed under both TZ=UTC and TZ=America/Chicago. Branch coverage is 99.02%.
• ruff check, ruff format --check, mypy, lint-imports, xenon, complexipy: pass.
• Checked by hand: get_continuous(time="2024-13-45"), get_daily(last_modified=["bad", None]), and ngwmn.get_water_level(datetime=[..., "nope"]) each raise with the argument named.

Not in scope (already true on main)

• A single string containing / is sent unchanged, so time="garbage/.." still reaches the service unvalidated. Validating each side would need to allow ISO 8601 start/duration intervals.
• Non-string elements (pd.Timestamp, datetime) fail with AttributeError/TypeError instead of a ValueError that names the argument.