feat(#833): error when forms reference CSV external datasets

[yasumorishima]

Description

Adds a form validation that fails when a form references an external dataset stored in a CSV file.

Since select_*_from_file support landed (medic/cht-core#10971), only XML datasets are supported. A form whose secondary instance uses src="jr://file-csv/..." uploads and loads without any error, but the question simply has no data. This catches that before the form reaches the server.

• src/lib/validation/form/no-csv-external-datasets.js: errors for every model/instance node whose src starts with jr://file-csv/. requiresInstance: false and skipFurtherValidation: false, as suggested in the issue. All offending sources are listed in a single message, with a link to the docs.
• test/lib/validation/form/no-csv-external-datasets.spec.js: 7 unit tests (no external data, XML source, one CSV source, several CSV sources, mixed XML/CSV, a jr://file-csv-backup/... near match, and a whitespace padded src).
• test/lib/validate-forms.spec.js: added the new validation to the expected load order.

No documentation change looked necessary: the docs already state that only XML files are supported. Note this is a behaviour change for existing configs that reference a CSV dataset - they will now fail validation instead of uploading a form that quietly shows no choices.

AI Disclosure: this PR was written primarily with Claude Code. It explored the codebase, drafted the validation following the existing ones and wrote the tests. I reviewed the result, verified the error message against the docs, and confirmed lint, unit tests (Node 20/22/24) and the e2e suite pass.

#833

Code review items

• Readable: Concise, well named, follows the style guide, documented if necessary.
• Documented: Configuration and user documentation on cht-docs
• Tested: Unit and/or integration tests where appropriate
• Backwards compatible: Works with existing data and configuration. Any breaking changes documented in the release notes.

License

The software is provided under AGPL-3.0. Contributions to this project are accepted under the same license.