Enable the provision of a model's description

## Feature Request

### Description of Problem
PyPI allows projects to include a one-line description, which is specified in the `pyproject.toml` file. Currently, the Rune generator does not support this feature, resulting in PyPI project pages displaying "No project description provided" (see [issue #4911](https://github.com/finos/common-domain-model/issues/4911) in the CDM project).

### Proposed Solution
The generator should accept a project description as input and populate the appropriate field in `pyproject.toml`.

#### Implementation Options
1. Accept a description file (e.g., `description.txt`) as input to the generator
2. Accept the description string directly as a generator parameter
3. Include the description as part of the model definition itself
4. Support manual override of the field in `pyproject.toml` after generation

Start by tracing the Rune generator's inputs and the code that produces pyproject.toml. Review how project metadata is currently assembled and determine which proposed description input fits the generator's design. Done means generated Python projects include the supplied one-line description in pyproject.toml and the behavior is covered by the relevant generator tests, if present.

Written by the indexing model from the issue text.