[ADD] website_sale_aplicoop: restore the test coverage lost with the dead code

The dead-code cleanup dropped 11 test files that were never wired into
tests/__init__.py. Reviewing what they covered turned up real holes, so the
worthwhile ones come back, rewritten against the current schema.

Blacklists were the serious gap: product, supplier and category exclusions
have absolute priority over product discovery and nothing exercised them.
The old file had two separate defects. Its supplier fixtures wrote
`main_seller_id` directly, but product_main_seller computes that field from
`variant_seller_ids`, so the compute reset it to False and the blacklist had
nothing to exclude; they now create real supplierinfo records. Worse, four
whole classes asserted against `group_order.product_ids` -- the m2m *input* --
instead of the discovery result, so they set `category_ids` and then checked a
field they never touched. Those go through `_get_products_for_group_order`
now, and three tests that had no assertions at all got some.

The remaining three failures were test bugs too, all Odoo 17->18 leftovers:

* Date cases assumed `pickup_date` derives from `start_date`. The chain is
  cutoff -> pickup -> delivery, and a recurring order whose start date has
  passed rolls forward to the current cycle, so a 2024 order has no 2024
  pickup. The new file anchors on future dates and finds the next 29 February
  dynamically, with a class documenting the roll-forward itself.
* `/eskaera/labels` is `type="json"`; the old test hit it with a plain GET and
  read the resulting 400 as a bug. It is called over JSON-RPC now, and a test
  pins the 400 so nobody repeats it. Also `uom.uom.categ` -> `uom.category`.
* `price_include` is computed in 18.0, so fixtures must set
  `price_include_override`. On top of that `_get_price` filters taxes by
  company and defaults to `env.company`, not the fixture's, which left the
  tax list empty -- `tax_included` was False for the wrong reason.

Two of the portal tests were passing for the wrong reason as well: the access
guard bounced them to /eskaera, which also answers 200. Membership has to be
set from the member side with `is_group`, and a new test checks the final URL
rather than the status alone. Each fixture that can silently build the wrong
thing now carries a guard test.

Left out on purpose: three files were unimplemented placeholders, and
test_draft_persistence still deserves recovering (see the notes file).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

website_sale_aplicoop/tests/__init__.py

@@ -19,3 +19,7 @@ from . import test_group_order_status_endpoint  # noqa: F401
 from . import test_home_delivery  # noqa: F401
 from . import test_forecasted_stock  # noqa: F401
 from . import test_online_payment  # noqa: F401
+from . import test_product_discovery  # noqa: F401
+from . import test_date_edge_cases  # noqa: F401
+from . import test_portal_routes  # noqa: F401
+from . import test_price_with_taxes_included  # noqa: F401