Index product prices for every active shop country

[boo-code]

Questions	Answers
Description?	When a customer's country has no specific tax rule for a product, sorting search/listing results by price returned 0 products for that customer, while every other sort worked. Root cause: indexProductPrices() computes prices for all countries but only inserts layered_price_index rows for the countries returned by the tax-rule query ($taxRatesByCountry) when PS_LAYERED_FILTER_PRICE_USETAX is on. The price sort joins that table with INNER JOIN ... AND psi.id_country = <customer country>, so a customer from a country that was never indexed matches no rows and gets an empty list. The fix indexes every active shop country: countries without a specific tax rule are indexed untaxed (0% rate) — exactly what the module already does for products that have no tax rule at all. Taxed countries keep their computed prices unchanged.
Type?	bug fix
BC breaks?	no
Deprecations?	no
Fixed ticket?	Fixes #1206.
How to test?	On a shop with at least two active countries where products have a tax rule for only one of them (e.g. FR), log in / set the context to the other country (e.g. US), open a listing and sort by price — before, the list is empty; after re-indexing prices, it shows the products. Verified on the default shop (active countries FR=8, US=21; products taxed for FR only): re-indexing product 1 keeps FR identical (price_min=22.944, price_max=28.680) and adds the US row untaxed (price_min=19.120, price_max=23.900), and the price-sort INNER JOIN on id_country=21 now returns the product instead of nothing.
Sponsor company

Verification (default shop, PS_LAYERED_FILTER_PRICE_USETAX on):

before:  layered_price_index for product 1 -> only id_country=8 (FR): 22.944 / 28.680
         price sort for US (id_country=21)  -> 0 products

after:   id_country=8  (FR): 22.944 / 28.680   (unchanged)
         id_country=21 (US): 19.120 / 23.900   (untaxed, newly indexed)
         price sort for US (id_country=21)  -> returns the product

indexProductPrices() is a private method on the module class (no unit-test harness covers it, like the rest of the indexer), so this was verified by re-indexing against a real database and diffing the rows, confirming the taxed-country prices are byte-identical. The index now covers active shop countries, with the same row shape the no-tax-rule path already produces.

[ps-jarvis]

Hello @boo-code!

This is your first pull request on ps_facetedsearch repository of the PrestaShop project.

Thank you, and welcome to this Open Source community!