Fix leaflux/fosberg test fixtures writing grid bands without index

[amarcozzi]

Summary

Follow-up to #548. The post-deploy CI test job on main went red on 4 TestListGridsWildcard::test_wildcard_sorting_matrix_returns_200 cases (assert 500 == 200) — not a product regression; the deploy itself succeeded and the feature is live.

Root cause

The leaflux and fosberg router-test grid_factory fixtures write source-grid documents straight to Firestore with bands shaped [{"key": key}] — missing the required index and type fields on Band.

CI runs the suite under pytest-xdist, and all tests share one test owner. While a leaflux/fosberg fixture grid is briefly alive on one worker, TestListGridsWildcard on another worker calls list_grids_cross_domain, which serializes every one of the owner's grids via Grid(**doc.to_dict()) (router.py:228) with no error handling. A fixture band with no index raises a Pydantic ValidationError → the endpoint returns 500 → the wildcard test fails.

Confirmed from prod logs: the failing input_values are exactly these fixtures ({'key': 'leaf_area_density'}, {'key': 'elevation'}, georef {'crs': 'EPSG:32611', 'shape': [6, 40, 40]}). The errors appeared only during the test session and stopped when the fixtures were torn down — the endpoint is healthy, and real API-created grids always have valid bands (this shape is only reachable by fixtures writing to Firestore directly).

Fix

Build the fixtures' bands with an enumerated index and type: "continuous" (every band these fixtures use — leaf_area_density, elevation, slope, aspect, irradiance.* — is continuous), so a concurrent cross-domain list serializes them cleanly:

data["bands"] = [
    {"key": key, "type": "continuous", "index": i}
    for i, key in enumerate(bands)
]

No product code changed. I deliberately left the list router alone — real data can't produce a bandless-index grid, so the fault is the fixtures, not the endpoint.

Verification

Ran leaflux + fosberg router tests and TestListGridsWildcard against a live server — 81 passed serially and 81 passed under 4-worker xdist (the parallel mode that triggered the flake). ruff check / format --check clean. Swept the suite for other fixtures with the same bandless-index pattern — none remain.