Adds an optional async hook on SourceAdapter so any adapter can surface a
settings-driven preview on its /adapters/<name> edit page. The framework
renders the result generically as a table — no adapter-name branches in
GUI templates or route code.
Framework changes:
- src/central/adapter.py: new async preview_for_settings(self, settings)
on the base class, default returns None. Adapters opt in by overriding;
non-overriding adapters render unchanged.
- src/central/gui/routes.py: GET /adapters/{name} instantiates the adapter
with a no-op _PreviewConfigStore stub and a /dev/null cursor path (GUI
has no live ConfigStore), constructs settings_obj via the schema, and
calls preview_for_settings inside a try/except. Result lands in template
context as preview_rows / preview_error.
- src/central/gui/templates/_adapter_preview.html: new partial. Generic
table with columns derived from the first dict's keys; error banner
mirrors the existing last_error article style.
- src/central/gui/templates/adapters_edit.html: one-line include between
the Region fieldset and Save/Cancel.
NWIS opt-in:
- New NWIS_MONITORING_LOCATIONS_URL constant and _PREVIEW_LIMIT cap of 50.
- preview_for_settings returns None when region is None, otherwise one-shot
fetches monitoring-locations within the bbox via a fresh aiohttp session.
Must work even when adapter is not started -- the GUI process never calls
startup(). Returns list[dict] with the contract column order: site_id,
name, site_type, state. Errors propagate so the framework can render the
operator-visible banner.
- HTTP call factored into _fetch_preview_text so tests mock cleanly.
Tests (7 new):
- tests/test_preview_hook.py: default returns None; partial renders list
with correct headers/rows/count; partial renders error banner; partial
renders empty when both context values are None.
- tests/test_nwis.py adds TestNWISPreview: returns None without region,
returns rows with correct column order, propagates HTTP errors.
Verification:
- 457/457 full suite green (was 450; +7 new tests).
- Live /adapters/nwis preview returns 50 rows with the contract keys
against the current production Iowa bbox.
- /adapters/eonet preview_for_settings returns None via base default --
proves framework is duck-typed, no NWIS-specific code in framework.
