### Check existing issues - [x] I've verified that this bug isn't described by any existing issues. ### Bug description With v4.2.0, Multi-base shorthand notation started to fail: `charmcraft.yaml`: ```yaml platforms: ubuntu@24.04:amd64: ``` ``` $ charmcraft pack charmcraft internal error: 1 validation error for PlatformCharm parts Value error, Platform 'ubuntu@24.04:amd64' declares a base in the platform's name and declares 'build-on' and 'build-for' entries. [type=value_error, input_value={'foobar-charm': {'s...'.', 'plugin': 'charm'}}, input_type=dict] For further information visit https://errors.pydantic.dev/2.12/v/value_error Full execution log: '/home/joakim/.local/state/charmcraft/log/charmcraft-20260330-145600.940980.log' ``` ### Steps to reproduce Provide step-by-step instructions to reproduce the bug. ### Environment We need to understand the context in which Charmcraft failed. Please let us know if you are running Charmcraft in destructive mode, whether you are using LXD or Multipass, and which operating system you are running Charmcraft on. ### charmcraft.yaml ```yaml If the issue is specific to a project, share its `charmcraft.yaml`. This field automatically renders as YAML, so you don't need to add special formatting. ``` ### Log output ```shell Provide any relevant logging. This field automatically renders as a log, so you don't need to add special formatting. ```