fix: use testmode boolean parameter for Mollie payments

Changed from mode: 'test' | 'live' to testmode: boolean as per Mollie
API requirements. The testmode parameter is set to true when the API
key starts with 'test_', false otherwise.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/providers/mollie.ts

@@ -18,10 +18,10 @@ const symbol = Symbol.for('@xtr-dev/payload-billing/mollie')
 export type MollieProviderConfig = Parameters<typeof createMollieClient>[0]
 
 /**
- * Determine the Mollie mode based on API key prefix
+ * Determine if testmode should be enabled based on API key prefix
  */
-function getMollieMode(apiKey: string): 'test' | 'live' {
-  return apiKey.startsWith('test_') ? 'test' : 'live'
+function isTestMode(apiKey: string): boolean {
+  return apiKey.startsWith('test_')
 }
 
 /**
@@ -168,8 +168,8 @@ export const mollieProvider = (mollieConfig: MollieProviderConfig & {
       validateProductionUrl(redirectUrl, 'Redirect')
       validateProductionUrl(webhookUrl, 'Webhook')
 
-      // Determine mode from API key (test_ or live_)
-      const mode = getMollieMode(mollieConfig.apiKey)
+      // Determine testmode from API key (test_ prefix = true)
+      const testmode = isTestMode(mollieConfig.apiKey)
 
       const molliePayment = await singleton.get(payload).payments.create({
         amount: {
@@ -179,7 +179,7 @@ export const mollieProvider = (mollieConfig: MollieProviderConfig & {
         description: payment.description || '',
         redirectUrl,
         webhookUrl,
-        mode,
+        testmode,
       } as any);
       payment.providerId = molliePayment.id
       // Use toPlainObject if available, otherwise spread the object (for compatibility with different Mollie client versions)