Fixes #1217. Problem When "platforms" is provided as a list instead of a mapping, "_vectorise_platforms()" attempts to call ".items()" before the input type is validated. This raises an "AttributeError" instead of a useful project validation error. Changes - Validate non-mapping "platforms" inputs using the existing Pydantic adapter before vectorisation. - Preserve the existing processing of valid platform mappings. - Add regression tests covering nonempty lists, empty lists and string inputs. Verification The targeted project-service unit tests, Ruff lint checks and formatting checks passed in the fork's GitHub Actions regression workflow. The broader QA and Spread workflows have not yet been confirmed passing. A separate OSV scan identified a vulnerable "certifi" dependency; that finding is outside the scope of this fix. --- - [x] I've followed the [contribution guidelines](https://github.com/canonical/craft-application/blob/main/CONTRIBUTING.md). - [x] I've signed the [CLA](http://www.ubuntu.com/legal/contributors/). - [x] I've successfully run `make lint && make test`. - [x] I've added or updated any relevant documentation. - [x] In documents I changed, I [added a meta description](https://canonical-starflow.readthedocs-hosted.com/how-to/add-a-page-meta-description/) if one was missing. - [x] I've updated the relevant release notes.