Refactor ERA5 CDS request-dict construction for unit testability

[ewquon]

Background

Surfaced during testing-coverage review of PR #197.

download_era5_day (ext/NumericalEarthCDSAPIExt.jl:162) and download_era5_multivar_day (line 347) each inline the construction of the CDS request dictionary (variable list, year/month/day/time strings, area, pressure_level). The shape is currently only validated end-to-end by hitting CDS — typos or off-by-one zero-padding errors only surface as opaque server 4xx responses.

A coverage-report inspection confirms this: at the current PR HEAD, NumericalEarthCDSAPIExt.jl is at ~62% line coverage with 90 missed lines, and the bulk of the uncovered lines sit inside these per-day download paths past the skip_existing early-return. Without a CDS-callable test, those lines are effectively unreachable from the unit suite.

Proposed refactor

Extract a pure helper:

build_era5_request(name_or_names, dataset, datetimes; region) -> Dict{String, Any}

…that both download_era5_day (line 162), download_era5_multivar_day (line 347), and the Metadatum overload (line 105) delegate to.

Tests this enables

Pure dict-shape assertions, no network:

• Single-level dataset: no pressure_level key
• Pressure-level dataset: pressure_level is sorted hPa strings
• BoundingBox region produces an area key in [N, W, S, E] order
• Column region produces an area key with the right ε
• Multiple datetimes on the same day produce a time array, not a single string
• Zero-padded month/day/hour ("02", not "2")

Why it's worth doing

1. Bug surface. The request shape is what CDS actually consumes; today a typo there is invisible until a server 4xx, with no useful local signal.
2. Coverage. This is the single biggest unit-testable chunk left in the CDS extension. Extracting and testing it should bump NumericalEarthCDSAPIExt.jl line coverage materially — most of the currently-uncovered ~90 lines live in the per-day paths that this refactor opens up to direct test.
3. Mostly mechanical. Pull the dict-build out, replace each inline construction with a call to the helper, leave behavior unchanged.

Suggested as a small follow-up PR after #197 lands.