FEAT: enable disabling branch coverage

[redeboer]

Closes #622

✨ New features

Added a --branch-coverage/--no-branch-coverage flag to the python subcommand, allowing repositories to opt out of branch coverage in the Coverage.py configuration. Branch coverage remains enabled by default, so existing setups are unaffected. The flag is configurable per-repository through

[tool.compwa.policy.python]
branch-coverage = false

in pyproject.toml, and a CLI override takes precedence over the pyproject.toml setting.

🖱️ Developer experience

• Add tests covering disabling branch coverage via _update_codecov_settings, the branch-coverage pyproject.toml key, the True default, and the CLI-overrides-pyproject.toml precedence.