feat: add discount_percentage attribute to product export feed

[ahuininga-orisha]

Magento merchants often want to use a discount percentage in Tweakwise for faceting, sorting, or display purposes (e.g. "Show products with 20%+ discount"). Until now, there was no ready-made attribute for this — customers had to set up complex custom logic on their side to derive it from the exported price fields.

This change adds a computed discount_percentage attribute to the Tweakwise product export feed. The value is calculated from the already-available price index data (no extra queries), emitted only when a real discount exists, and skipped for bundle products where the pricing model is too complex to reliably reduce to a single percentage.

Changes

File	What changed
Model/Write/Products/CollectionDecorator/DiscountPercentage.php	New decorator that computes and emits the discount_percentage attribute after the price step
Model/Write/Products/CollectionDecorator/Price.php	Stores regular_price (exchange-rate-applied price_index.price) before getPriceValue overwrites the price key, making it available to downstream decorators
etc/di.xml	Registers DiscountPercentage after price in both the full export (Products\Iterator) and price-only export (Price\Iterator) decorator pipelines

Formula: discount_percentage = round((regular_price - final_price) / regular_price × 100)

Known limitations (by design):

• Bundle products: skipped — pricing model too complex
• Customer group prices: uses guest price group (pre-existing export limitation)
• Cart/quote price rules: not reflected (Magento limitation — only catalog price rules and special prices are in the price index)

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How to test

Scenario 1 — Discount percentage appears for discounted simple products

1. In Magento Admin, set a special price (e.g. €24 on a product with regular price €32) on a simple product.
2. Reindex: bin/magento indexer:reindex catalog_product_price
3. Run bin/magento tweakwise:export --file /tmp/test_export.xml
4. Open /tmp/test_export.xml and find the product by SKU.
5. Verify it contains <name><![CDATA[discount_percentage]]></name> with <value>25</value> (for 32→24 = 25%).

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Scenario 2 — No discount_percentage for full-price products

1. Find a product with no special price and price_index.final_price == price_index.price.
2. Run the export as above.
3. Verify that product's <item> block contains no discount_percentage attribute.

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Scenario 3 — Bundle products are excluded

1. Find a bundle product in the feed.
2. Verify its <item> block contains no discount_percentage attribute, even if child products have discounts.

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Scenario 4 — Price-only export also includes discount_percentage

1. Run bin/magento tweakwise:export --type price --file /tmp/test_price_export.xml
2. Find a discounted simple product in the output.
3. Verify discount_percentage is present and correct.

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Scenario 5 — Exchange rate stores show correct percentage

1. Configure a store view with a currency exchange rate (e.g. 2.237).
2. Run the full export.
3. Find the same discounted product in the store's section of the feed.
4. Verify the discount_percentage value matches the expected ratio (same percentage as the base store, since both regular and final price are scaled equally).

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