Remove testmode config and pre-checkout customer creation

[sandervanhooft]

Changes

Issue #5: Remove VATLY_TESTMODE

The Vatly API infers testmode purely from the API key prefix (test_ = test mode). No need to configure or pass it explicitly.

Changes:

• Removed testmode config option from config/vatly.php
• Changed VatlyConfig::isTestmode() to derive from API key prefix instead of config
• Removed ManagesTestmode trait entirely
• Removed use ManagesTestmode from CheckoutBuilder and SubscriptionBuilder
• Removed testmode from checkout payload
• Updated tests to reflect payload changes

Issue #6: Remove ensureHasVatlyCustomer from checkout/subscribe

Don't pre-create customers before checkout. Let Vatly handle customer creation during checkout, then sync via webhooks. This fixes the 422 error when email already exists in Vatly.

Changes:

• Removed $this->ensureHasVatlyCustomer() call from checkout() method
• Removed $this->ensureHasVatlyCustomer() call from subscribe() method
• customerId in payload will be null for new users (filtered out by array_filter)

Closes #5
Closes #6