[SNAPWOO-76] Snapchat for WooCommerce shows low transaction ID overlap between Pixel and CAPI purchase events

[jamesmorrison]

Changes proposed in this Pull Request:

Closes https://linear.app/a8c/issue/SNAPWOO-76/snapchat-for-woocommerce-shows-low-transaction-id-overlap-between

Screenshots:

N/A

Detailed test instructions:

Test 1: Pixel PURCHASE event uses order ID

1. Add a product to your cart and complete checkout.
2. On the order confirmation (thank you) page, open browser dev tools Console.
3. Search the page source or console for snaptr("track", "PURCHASE" -- you can also check the inline script on the page.
4. Verify the payload contains:

• event_id = the WooCommerce order ID (a numeric string like "123")
• client_dedup_id = same value as event_id
• transaction_id = same value as event_id

5. All three should be identical and match the order ID shown on the thank-you page.

Test 2: CAPI PURCHASE event uses order ID

1. On the same order confirmation page, check the Network tab for requests to admin-ajax.php or look at server logs / Action Scheduler for the queued snapchat_wc_send_conversion_event action.
2. The CAPI payload should have:

• event_id = the same WooCommerce order ID string (e.g. "123")
• custom_data.order_id = the same value

3. Confirm event_id matches the Pixel's transaction_id from Test 1.

Test 3: Duplicate tracking guard still works

1. After completing Test 1, reload the order confirmation page.
2. Check the page source -- the snaptr("track", "PURCHASE" call should not appear a second time (blocked by the _snapchat_pixel_tracked meta flag).

Test 4: Non-purchase events are unaffected

1. Visit a product page -- check that VIEW_CONTENT pixel event still fires with a UUID-style event_id (not an order ID).
2. Add a product to cart -- check that ADD_CART pixel event still fires with a UUID-style event_id.
3. Go to checkout -- check that START_CHECKOUT pixel event still fires with a UUID-style event_id.
4. Visit any non-product/non-checkout page -- check that PAGE_VIEW still fires normally.

Additional details:

Fix - Fix low transaction ID overlap by using order ID as event_id for purchase deduplication.