waterdata: tighten stats + OGC pagination (geometry, KeyError, non-200)

[thodson-usgs]

Summary

Three correctness fixes to the two pagination loops in `dataretrieval.waterdata.utils`.

• `get_stats_data` dropped geometry on continuation pages. Page 1 honored `GEOPANDAS` but pages 2..N hard-coded `geopd=False`. With geopandas installed, a multi-page stats response started as a `GeoDataFrame` and pages 2..N came back as plain `DataFrame`s; `pd.concat` then silently downgraded the result and the caller lost geometry / CRS. Use `geopd=GEOPANDAS` on every page.
• `get_stats_data` raised `KeyError` on responses without a `next` key. Some terminal responses simply omit it. Switched to `body.get("next")`, which produces `None` and exits the loop cleanly.
• Both pagination loops swallowed non-200 continuation pages silently. A 4xx/5xx page whose body happened to JSON-decode could be appended as data and pagination would quietly stop — the caller got a partial result with no warning. Added an explicit `if status_code != 200` raise inside each loop so the existing log-and-truncate handler fires deliberately rather than incidentally.

Test plan

• Three new regression tests in `tests/waterdata_utils_test.py`: `_walk_pages` raises and stops on a 500-status continuation; `get_stats_data` accepts a response without a `next` key; `get_stats_data` truncates cleanly on a 503 continuation.
• Existing tests in the file still pass.
• Full local test suite passes (229 tests, with `API_USGS_PAT` set).