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/services/provisioning-service.ts

@@ -20,12 +20,14 @@ export class ProvisioningService {
     db: D1Database,        // cloud-instances-db: pricing, providers
     userDb: D1Database,    // telegram-conversations: users, deposits, orders
     linodeApiKey?: string,
-    vultrApiKey?: string
+    vultrApiKey?: string,
+    linodeApiUrl?: string, // Optional: for testing with emulator
+    vultrApiUrl?: string   // Optional: for testing with emulator
   ) {
     this.env = env;
     this.repo = new ProvisioningRepository(db, userDb);
-    this.linodeProvider = linodeApiKey ? new LinodeProvider(linodeApiKey) : null;
-    this.vultrProvider = vultrApiKey ? new VultrProvider(vultrApiKey) : null;
+    this.linodeProvider = linodeApiKey ? new LinodeProvider(linodeApiKey, linodeApiUrl) : null;
+    this.vultrProvider = vultrApiKey ? new VultrProvider(vultrApiKey, vultrApiUrl) : null;
   }
 
   /**