feat(order-endpoint): Add BN code to confirm_payment_source requests (6099)

[Narek13]

Issue Description

confirm_payment_source was the only method in OrderEndpoint not explicitly setting the PayPal-Partner-Attribution-Id header, causing gaps in PayPal partnership reporting and revenue attribution.

PR Description

Adds the BN code header to confirm_payment_source requests in OrderEndpoint, aligning it with the established pattern used in all other API methods (create, capture, authorize, order, patch).

if ( $this->bn_code ) {
    $args['headers']['PayPal-Partner-Attribution-Id'] = $this->bn_code;
}

Acceptance criteria covered

• All confirm_payment_source requests include the PayPal-Partner-Attribution-Id header
• No regression in existing payment method behaviour (Multibanco, OXXO, etc.)

[github-actions]

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What's included:

• ✅ WordPress (latest)
• ✅ WooCommerce (latest)
• ✅ PayPal Payments plugin v4.0.2-pr4247-23852576447-gabaa587 (built from this PR)

Login credentials:

• Username: admin
• Password: password

Plugin Details:

• Version: 4.0.2-pr4247-23852576447-gabaa587
• Commit: fd8d2cf
• Artifact: woocommerce-paypal-payments-4.0.2-pr4247-23852576447-gabaa587

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