cmip7: prefer variant variable term over root for ATTR004 long_name

[JanStreffing]

Sister fix for WCRP-CMIP/WCRP-universe#191.

Several CMIP7 variables have tile-specific or operation-specific variants in the variable data descriptor (cveggrass / cvegshrub / cvegtree, hursmin / hursmax / tasmincrop, gpp/npp/ra/rh × grass/shrub/tree, ...). All variants share the same variable_id global attr (e.g. variable_id = "cVeg" for all four cVeg variants).

The current lookup in plugins/cmip7/cmip7.py picks the bare root term:

var_terms = find_terms_in_data_descriptor(
    expression=var_id_lower,
    data_descriptor_id="variable",
    selected_term_fields=["long_name"],
)
if var_terms:
    for term in var_terms:
        if getattr(term, "id", None) == var_id_lower:
            expected_var = term
            break

After WCRP-universe#191 generalizes the root's long_name and populates per-variant long_names on cveggrass/cvegshrub/cvegtree (etc.), this plugin still needs to pick the right candidate. Picking the root means the ATTR004 long_name check emits a MEDIUM finding on every non-root variant because the file's variant-specific long_name doesn't match the generic root. ATTR004 long_name is advisory per CMIP7 guidance (see WCRP-universe#190 discussion), so this PR is accuracy of the warning rather than a publication unblock.

This PR takes the variant whose registered long_name matches the file's long_name attr, and falls back to the root term if no variant matches. Single-value comparison semantics for the check stay the same. The resolution just picks the right term.

No test infra changes. The existing ATTR004 path covers both branches.