tests/ngwmn_test.py is fully live; move NGWMN tests offline per AGENTS.md

[thodson-usgs]

Summary

tests/ngwmn_test.py (added in #324) is a file of tests that hit the live NGWMN OGC API on every run, rather than mocking with httpx_mock + tests/data/ fixtures.

The convention it breaks

AGENTS.md → Testing Gotchas:

Tests mock HTTP with pytest-httpx's httpx_mock fixture and fixtures under tests/data/; keep new API tests offline.

The doc treats live tests as named exceptions (e.g. tests/nwis_test.py::test_nwis_service_live) and reiterates under Conventions: "tests should mock with httpx_mock." tests/ngwmn_test.py is instead an entire file of live tests — its module docstring says "These hit the live NGWMN OGC API …"; every getter test (get_sites/get_water_level/get_providers/get_lithology/get_well_construction) makes real requests, and test_state_queryables_still_diverge_upstream does a direct httpx.get(.../queryables).

Why it matters

• CI-fragile: these fail on transient USGS outages — feat: add NGWMN getters on a shared, API-agnostic OGC engine #324's CI already went red on live 503s unrelated to the code change.
• Slow & non-deterministic, and they can't run in an offline/sandboxed CI.

Scope / tension

Not unique to NGWMN: tests/waterdata_test.py is also substantially live (only a minority use httpx_mock), so the "offline" rule isn't uniformly enforced today. Worth picking a direction and applying it consistently.

Proposed

1. Add offline httpx_mock-based tests for the five NGWMN getters using recorded GeoJSON response fixtures under tests/data/ (mirroring mocked cases like tests/waterdata_test.py::test_mock_get_samples).
2. If a live check is still wanted, keep at most a small, explicitly-named/gated live smoke test (like test_nwis_service_live); the test_state_queryables_still_diverge_upstream pin can remain live but be marked as the deliberate live exception.
3. (Optional, separate) extend the same offline treatment to the live Water Data getter tests for consistency.

Raised from a code review of #324.