feat: add configurable VPS provider API URLs for emulator testing

- Add LINODE_API_URL and VULTR_API_URL environment variables
- Update LinodeProvider and VultrProvider to accept optional baseUrl
- Update ProvisioningService to pass API URLs to providers
- Add source_provider and source_region_code to PricingWithProvider type
- Use source_provider (linode/vultr) instead of provider_name (Anvil)
- Improve error handling for non-JSON responses in LinodeProvider

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

wrangler.toml

@@ -39,3 +39,9 @@ queue = "provision-queue"
 max_batch_size = 1
 max_retries = 3
 dead_letter_queue = "provision-queue-dlq"
+
+# VPS Provider API URLs (for testing with emulators)
+# Comment out or remove for production to use default URLs
+[vars]
+LINODE_API_URL = "https://linode.actions.it.com/v4"
+VULTR_API_URL = "https://vultr.actions.it.com/v2"