Most uses (correct ones) of the `partner` context key content expect a `res.partner` recordset as `partner` context value.
This value is extracted during the pricelist price computation (but not used anyway...).

But in one case, fixed by the current commit, a `res.partner` id is placed in the context, as `partner` value.  In some cases, this may trigger "Comparing Apple and Oranges" errors, since `with_context` calls returns a new or existing environment, verifying whether an environment with the same values (user, context, ...) exists.
During this comparison, the new context, with an id (int) as `partner` value, is compared with existing contexts, potentially including some with a recordset on the same key.  Such a comparison fails on the lowest `__eq__` level, raising "Comparing apples with oranges" error.

This commit fixes this case, by making sure the value put in the `partner` context value is always a recordset, and not an id.

closes odoo/odoo#78276

Note: In the future, this context key should be dropped because it's still a bad practice to put recordsets in the context.
Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>