feat: Add product count fetch for filter buttons

[ahuininga-orisha]

Summary

• Mirrors the filter count feature from EmicoEcommerce/Magento2Tweakwise#372 for the Hyvä compatibility layer.
• On initial page load, the "Show X items" button now displays the actual product count for the current category/search context instead of the generic "Show items" label.
• On every filter checkbox change (when formFilters is enabled), a lightweight AJAX request is fired to tweakwise/ajax/productcount and all "Show X items" buttons are updated without triggering a full navigation reload.
• When AJAX filters are enabled and a navigation response is returned, the button count is also updated from the product_count field included in the response (added in #372).
• In-flight count requests are aborted when a new filter change fires, preventing stale count updates.

How to test

Setup

1. Enable both tweakwise/layered/enabled and tweakwise/layered/form_filters in Stores → Configuration → Tweakwise.
2. Make sure the branch from EmicoEcommerce/Magento2Tweakwise#372 is active in vendor/tweakwise/magento2-tweakwise.
3. Navigate to a category page with Tweakwise facets, e.g. /women/tops-women/.

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Scenario 1 — Initial page load

1. Open the category page.
2. Each "Show X items" button should display the actual product count for that category, not the generic "Show items" label.

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Scenario 2 — Single filter checkbox

1. Check one filter value (e.g. size M).
2. Without clicking the button, observe all "Show X items" buttons update to reflect the new count.
3. Verify the count matches what Tweakwise returns for that category + filter combination.

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Scenario 3 — Multiple filters combined

1. Check a second filter value (e.g. color Gray) while size M is still checked.
2. All buttons should update to the combined count (category + size M + color Gray).
3. Uncheck one filter; buttons should update back to the count for the remaining filter only.

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Scenario 4 — Full filter apply still works

1. Check a filter and click the "Show X items" button.
2. The page should navigate/reload with the filter applied as before.

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Scenario 5 — formFilters disabled

1. Disable tweakwise/layered/form_filters.
2. Verify no count AJAX requests are fired on filter changes (the feature is inactive).