Add support for PEP735-style dependency groups

### What is the problem or limitation you are having?

Currently, Briefcase ignores [PEP-735](https://peps.python.org/pep-0735/)-style dependency groups.

[PEP-735](https://peps.python.org/pep-0735/) added support for dependency groups that are not published to PyPI and is now the standard way that [PDM](https://pdm-project.org/latest/usage/dependency/#add-development-only-dependencies) and [uv](https://docs.astral.sh/uv/concepts/projects/dependencies/#adding-dependencies) adds dependencies. Briefcase might want to support using those for test dependencies as well.

By default, these tools use the `dev` dependency group, but users can also specify a specific group name, like `test` to store the dependencies in as well.

### Describe the solution you'd like

Supporting dependency groups as test dependencies in Briefcase.

### Describe alternatives you've considered

Don't support it

### Additional context

_No response_

No files, tests, or entry points are named. Start by locating how Briefcase currently reads test dependencies and compare that behavior with PEP 735 dependency-group semantics, including the default dev group and user-selected group names. Done means dependency groups can be used as test dependencies without publishing them to PyPI.

Written by the indexing model from the issue text.