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>

src/handlers/provision.ts

@@ -114,7 +114,9 @@ export async function handleProvision(
       env.DB,       // cloud-instances-db
       env.USER_DB,  // telegram-conversations
       env.LINODE_API_KEY,
-      env.VULTR_API_KEY
+      env.VULTR_API_KEY,
+      env.LINODE_API_URL,
+      env.VULTR_API_URL
     );
 
     // Provision server