Fixes #750 A platform given as a string or list (e.g. `my-platform: amd64`) crashed with an AttributeError, since both `_vectorise_platforms` and `Platform._validate_platform_set` call `.get()` on it. Both now pass non-mappings through so pydantic reports a normal validation error for `platforms.my-platform`. Added string and list cases to `test_get_platforms_bad_value`. I ran ruff, ty, codespell and the full test suite in a Linux container rather than `make lint && make test`. No new failures: the tests that fail there fail on main too. --- - [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/). - [ ] I've successfully run `make lint && make test`. - [ ] I've added or updated any relevant documentation. - [ ] 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.