Add plone.distribution support

[ericof]

What

Adds plone.distribution support to pytest-plone: fixtures to create a Plone site from a distribution in tests.

New fixtures

Fixture	Scope	Description
create_site	session	Callable func(app, answers) -> PloneSite that creates a new site from a distribution, deleting any existing site with the same site_id first, and sets it as the current local site.
distribution_name	session	Name of the distribution to create sites from (default "testing"). Override in your conftest.py.
answers	session	Mapping of answers passed to the distribution handler (site id, title, language, ...). Override to customize.
site_logo	session	Data-URI logo used as the site_logo answer.
site_owner_name / site_owner_password	session	Accessors for SITE_OWNER_NAME / SITE_OWNER_PASSWORD, so tests and fixtures can depend on them instead of importing the constants.

create_site uses the private plone.distribution.api.site._create_site to avoid committing, so the testing layer still isolates each test. The created site coexists with the site provided by the testing layer (a second site, by design).

Types

Adds a SiteCreator Protocol in _types.py (callable-fixture return type, following the existing convention).

Dependencies

• Runtime: plone.distribution, plone.exportimport
• Test: plone.app.caching, plone.volto — required because the testing distribution from plone.distribution.testing (used to exercise create_site) applies their GenericSetup profiles.

Testing

• New tests/fixtures/test_distribution.py: default-value checks for the helper fixtures, and create_site functional tests (site created + idempotent re-create) against the testing distribution loaded from plone.distribution.testing.
• Full suite: 82 passed
• uvx mypy src clean, ruff clean
• news/58.feature added

Closes #58