Upgrade pytest-plone to 1.1.0

[ericof]

What

Upgrade pytest-plone to >=1.1.0 and adapt the test layer setup to the new release.

GitLab issue: https://gitlab.kitconcept.io/kitconcept/distribution-kitconcept-intranet/-/issues/462

Why

pytest-plone 1.1.0 changed fixtures_factory to default to keep_session=True, which pins every registered layer's session-scoped fixture for the whole test session instead of tearing it down around each function-style test.

Because our conftest.py registers the acceptance, functional and integration layers, this kept them all set up simultaneously — and exposed a latent Plone site id collision:

• The acceptance layer pulls in the stock plone.app.testing.PLONE_FIXTURE transitively, via plone.app.robotframework's REMOTE_LIBRARY_BUNDLE_FIXTURE (bases=(PLONE_FIXTURE,)).
• The stock PLONE_FIXTURE.setUpDefaultContent creates a Plone site with the default id plone.
• Our own CoreFixture also creates a site with site_id="plone".

With both sessions kept alive at once, the second site creation failed with BadRequest: The id "plone" is invalid - it is already in use.

How

• Bump the pytest-plone pin to >=1.1.0 in backend/pyproject.toml (+ uv.lock).
• Add backend/src/kitconcept/core/testing/robot.py exposing RF_LIBRARIES (the robot remote-library tuple).
• Rebuild REMOTE_LIBRARY_BUNDLE_FIXTURE locally with bases=(kitconcept_FIXTURE,) so it rides on our kitconcept site instead of the stock PLONE_FIXTURE. RemoteLibraryLayer itself creates no site — it only attaches remote libraries to PloneSite — so re-parenting is safe.
• Keep the faster keep_session=True.
• Rename the distribution fixture to distribution_name and make create_site idempotent (drop a pre-existing site id before recreating).

Test plan

• make test / uv run pytest in backend/ — 288 passed.

Local Results

x5.8 improvement:

• pytest-plone 1.0.0: 288 passed in 66.38s (0:01:06)
• pytest-plone 1.1.0: 288 passed in 11.40s