t523d: Add payee.merchant_id to purchase_units in create_order()

[superdav42]

Summary

• Adds payee.merchant_id to purchase_units[0] in create_order() when $this->merchant_id is set (OAuth/PPCP mode)
• PayPal partner model requires this field to route payments to the correct merchant account; previously only the PayPal-Auth-Assertion header was set
• Field is conditionally added — manual-credentials mode (no merchant_id) is unaffected

Changes

inc/gateways/class-paypal-rest-gateway.php

• Extracted purchase_unit array into a local variable before building $order_data
• Added conditional payee.merchant_id injection when $this->merchant_id is non-empty

tests/WP_Ultimo/Gateways/PayPal_REST_Gateway_Test.php

• Added create_order_test_fixtures() helper to create user/customer/membership/payment
• Added invoke_create_order_and_capture() helper using wu_paypal_rest_order_data filter to capture order data before the API call
• Added test_create_order_includes_payee_merchant_id_when_set — asserts payee.merchant_id present in OAuth mode
• Added test_create_order_omits_payee_when_merchant_id_empty — asserts payee absent in manual-credentials mode

Test Results

OK (94 tests, 162 assertions, 5 skipped)

Closes #730

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📥 Commits

Reviewing files that changed from the base of the PR and between 03bdae6 and 2dc7a99.

📒 Files selected for processing (1)

• tests/WP_Ultimo/Gateways/PayPal_REST_Gateway_Test.php

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch bugfix/730-paypal-merchant-id

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Performance Test Results

Performance test results for f52793e are in 🛎️!

Note: the numbers in parentheses show the difference to the previous (baseline) test run. Differences below 2% or 0.5 in absolute values are not shown.

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Run	DB Queries	Memory	Before Template	Template	WP Total	LCP	TTFB	LCP - TTFB
0	40	37.73 MB	862.00 ms (+55.00 ms / +6% )	167.50 ms (-4.50 ms / -3% )	1068.50 ms (+51.50 ms / +5% )	1996.00 ms	1903.60 ms	91.85 ms (+2.65 ms / +3% )
1	56	49.02 MB	911.50 ms (-33.50 ms / -4% )	147.00 ms	1059.50 ms (-36.00 ms / -3% )	2030.00 ms	1949.60 ms	82.25 ms