chore: misc cleanups from the package review (bbox docs, nwis, empty-list)

[thodson-usgs]

Summary

A bundle of small, low-risk cleanups from the package review — two docs, three minor code fixes. No behavior change beyond the two noted.

#	Fix	Kind
1	bbox docstring: the 4th coordinate was labeled "Northern-most longitude"; for [xmin, ymin, xmax, ymax] it's the Northern-most latitude. Fixed across the OGC getters + get_samples (7 spots).	docs
2	get_field_measurements properties listed daily/continuous columns it doesn't have (time_series_id, statistic_id, time); now points to the field-measurements OpenAPI schema.	docs
3	nwis.get_discharge_peaks ran format_response twice (via _read_rdb, then again with service="peaks"); parse raw with read_rdb so it formats once. Behavior-preserving (the first pass was a no-op/redundant for peaks).	fix
4	nwis.query_waterdata validated a dead state_cd key while the docs, error message, and callers all use stateCd; aligned so a stateCd-only query is recognized as a major filter (this becomes load-bearing once #310 stops the always-inject that currently masks it).	fix
5	_construct_api_requests emitted an empty-list param as a filterless &param= (read by the server as "match empty"); now omitted.	fix

Verification

• bbox: 0 remaining "Northern-most longitude", 7 "Northern-most latitude".
• empty list: parameter_code=[] → omitted; parameter_code=["00060"] → present (regression test added).
• query_waterdata("peaks", stateCd="HI") now passes the major-filter check; a no-filter call still raises TypeError.
• Full nwis suite (47) passes; ruff clean.

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