feat: add a unified county argument, like state

[thodson-usgs]

TL;DR: Adds a county argument that works like state. It accepts a FIPS code, or a name with its state, and sends each service the fields that service filters on. I checked every conversion against the live services. Scope follows state: Water Data's location and statistics getters, ngwmn.get_sites, and nwdc.get_wateruse, but not WQP, NWIS, or Samples. ADR 0013 and CONTEXT.md now record the rule both arguments follow.

Stacked on #432 (Virgin Islands name fix): its commit e3c3c335 is the first one here. Merge #432 first; review the commits after it.

Changes

• codes.counties (new; exposes to_county and apply_county). The table holds 3,239 counties and county equivalents. It is a snapshot of the Water Data counties collection for the states and territories that codes.states covers, and a live test compares the two. Accepted inputs:

  • "55025", 55025, or "US:55:025";
  • with state: "025", "Dane County", "dane", or "St Louis County" (case and periods don't matter, and the designation such as County or Parish is optional).

• Getters. In every getter below, state names the state the counties are in when county is given, instead of adding its own filter. Passing county together with the getter's own state or county parameter raises ValueError.

  Getter	Sent as
  waterdata.get_monitoring_locations, get_combined_metadata	state_code + county_code; counties in several states become a filter of OR'd pairs
  waterdata.get_stats_por, get_stats_date_range	county_code=US:55:025 (repeated)
  ngwmn.get_sites	state_name + county_name
  nwdc.get_wateruse	countyCd:55025 (the existing county now accepts every form above)

• Bug fix (NGWMN): get_sites(county_name=[...]) with more than one name returned no rows, because NGWMN matches nothing for a comma-joined county_name. Multi-value sites filters are now POSTed as CQL2.

• Docs: ADR 0013 gains a clause allowing a unified argument (an argument dataretrieval names for a concept the services spell differently) under four conditions:

  • the service's own parameters stay;
  • the conversion is an exact table, checked live against the service's reference collection;
  • combining the argument with the native parameter raises;
  • the docstring names the field it is sent as.

  CONTEXT.md defines State and County as domain terms and Unified argument as a core term.

Design choices for the maintainer

1. Names need a state. County names repeat across states (there are 30 Washington Counties), so a name without state raises. A bare name that matches two counties in one state also raises and lists both. This happens 12 times, e.g. Fairfax County and Fairfax City in Virginia.
2. Counties in several states, Water Data: these are sent as (state_code='55' AND county_code='025') OR …. Separate state_code and county_code lists would also match code 031 in Wisconsin. The request planner already splits a filter at its top-level OR. The catch is that county from more than one state can't be combined with your own filter, and the call raises instead, with a remedy.
3. NGWMN takes one state per call. OR'ing pairs needs a CQL text filter, and NGWMN's edge blocks those (HTTP 403).
4. NGWMN and Connecticut. NGWMN still files Connecticut sites under the eight counties the state replaced with planning regions in 2022, and the regions don't map one-to-one onto the old counties. get_sites(county="09110") therefore raises and points to county_name="Hartford County". NGWMN's Municipality of Anchorage is handled with a one-line alias. A live test fails if NGWMN changes either.
5. NWDC behavior change: get_wateruse(state=..., county=...) used to raise for naming two locations. Now state qualifies the county. An unknown county, such as Connecticut's retired 09003, raises locally instead of returning HTTP 400.
6. The table is a separate module, _county_table.py, so that the doubled-word hook can skip it ("Walla Walla County").

Verification

• Conversions (offline): every one of the 3,239 entries round-trips through each form: full name, lower-case name, 3-digit code with state, 5-digit string, US: form, and integer. Each bare name resolves unless it's one of the 12 ambiguous ones.
• Live, 69 counties: one random county per state and territory, plus edge cases (Virginia independent cities, Baltimore and St. Louis cities, Alaska boroughs and census areas, a Louisiana parish, Puerto Rico municipios, Connecticut planning regions, DC, Guam, American Samoa, the Northern Mariana Islands, and the US Virgin Islands).
  • Every input form round-trips.
  • get_monitoring_locations returns only rows with the requested state_code, county_code, and county_name.
  • ngwmn.get_sites returns only the requested state_name + county_name, or raises for Connecticut.
  • The statistics service and NWDC accept every code.
  • The NWDC wateruse model has no data for Alaska, Hawaii, or the territories, and responds with HTTP 400; state="AK" does the same today.
• Live, at scale: county= with all 174 Wisconsin and Illinois counties returns the same 5,136 stream sites as state=["WI", "IL"], in 2 requests.
• Live tests: the 4 new ones pass: the county table against the Water Data counties collection, NGWMN's county names, and the state table against the states collection (moved here from fix(codes): name the Virgin Islands as the services do #432, because ADR 0013's new clause cites it).
• Offline: 1272 passed, branch coverage 98.9%+. ruff, mypy --strict, import-linter, xenon, and complexipy pass, and so do the pre-commit hooks. The Sphinx build (notebooks not executed) adds no warnings.