feat: source portal DataStore fallback for harvested datasets

[aborruso]

Problem

When a dataset is harvested by an aggregator portal (e.g. dati.gov.it), the ckan_list_resources tool reports DataStore: No even when the DataStore is active on the source portal.

Real example

Querying "Rilevazione qualità aria 2025" (Comune di Milano) on dati.gov.it:

• ckan_list_resources → DataStore: No
• Same dataset on dati.comune.milano.it → DataStore: Yes ✓

The resource download URL already contains the source portal domain, dataset ID, and resource ID:

https://dati.comune.milano.it/dataset/f02f7e96.../resource/9010ac16.../download/...

So the information needed to check the source portal is already available in the metadata.

Proposed Solution

When datastore_active is false/null on a resource, inspect the download URL. If the domain differs from server_url, attempt a DataStore lookup on the source portal using the extracted resource ID, and report the result alongside the original resource metadata.

New optional parameter check_source_portal (boolean, default true) on ckan_list_resources.

New fields in response: source_datastore_active, source_portal_url.

Spec

Full proposal and scenarios in OpenSpec change:
openspec/changes/add-source-portal-datastore-fallback/

• proposal.md
• tasks.md
• spec delta