[FIX] stock: differentiate model in stock_forecasted

When using browser's "history-back" event from a product.product or
product template form view to the forecast report, active_model
previously defined in context is lost and defaults to 'product.template'
but with the previous ID, which can result in showing the forecast of a
template with the id of a product product. It can even give nothing if
the id doesn't exists for the product template model.

closes odoo/odoo#107082

Task: 2985735
Signed-off-by: William Henrotin (whe) <whe@odoo.com>