t523e: Log PayPal-Debug-Id response header from API calls

[superdav42]

Summary

PayPal requires debug IDs from test API calls for the review submission. The PayPal-Debug-Id response header should be captured and logged from every API response.

File

inc/gateways/class-paypal-rest-gateway.php — api_request() method (or Base_PayPal_Gateway)

Acceptance Criteria

• PayPal-Debug-Id extracted from every API response via wp_remote_retrieve_header()
• Debug ID logged alongside existing API response logging

Part of #725

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[superdav42]

already done in #726

[superdav42]

Closing Summary

What was done:

• Added PayPal-Debug-Id response header capture in api_request() via wp_remote_retrieve_header()
• Logged at info level with method+endpoint context: PayPal-Debug-Id: <id> [POST /v2/checkout/orders]
• Included in error log messages: API Error (400): ... [Debug-Id: <id>]
• Exposed in WP_Error data array under debug_id key for callers

Testing Evidence: self-assessed — logging-only change using standard WP function. No business logic altered, no conditional paths changed.

Key decisions:

• Used HEAD version's log format (includes method+endpoint) which provides better context than the simpler format in the original commit
• Kept debug_id in WP_Error data array so callers can surface it if needed

Files changed: inc/gateways/class-paypal-rest-gateway.php (+4, -3 net after rebase)

Blockers: None

Follow-up: None required

PR: #732 (merged at 7abeac2)

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