Fix gateway_customer_id type validation in membership-update API schema

[vuckro]

Problem

The gateway_customer_id field in inc/api/schemas/membership-update.php was defined as integer type, causing API validation errors when updating memberships with Stripe customer IDs.

Symptoms

• Membership update API requests failing with type validation errors
• Stripe customer IDs are strings (e.g., cus_1234567890abcdef) but schema expected integers
• Payment gateway integrations broken due to type mismatch

Root Cause

File: inc/api/schemas/membership-update.php (line ~47)

'gateway_customer_id' => array(
    'description' => __('Gateway Customer ID.', 'wp-ultimo'),
    'type'        => 'integer', // ❌ Incorrect - causes validation failures
),

Most payment gateways (Stripe, PayPal, etc.) use alphanumeric string IDs, not integers.

Solution

Changed the field type from integer to string:

'gateway_customer_id' => array(
    'description' => __('Gateway Customer ID.', 'wp-ultimo'),
-   'type'        => 'integer',
+   'type'        => 'string',
),

Benefits

• ✅ Stripe compatibility: Accepts customer IDs like cus_1234567890abcdef
• ✅ Gateway flexibility: Works with PayPal, Square, and other processors
• ✅ Backward compatibility: PHP auto-converts existing integer IDs to strings
• ✅ API standards: Aligns with REST API best practices

Testing

• Verified with real Stripe customer IDs
• Tested membership update endpoints
• Confirmed backward compatibility with existing data
• No breaking changes detected

Type of Change

• Bug fix (non-breaking change which fixes an issue)

[superdav42]

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