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/vultr-provider.ts

@@ -36,10 +36,12 @@ interface VultrError {
 }
 
 export class VultrProvider extends VPSProviderBase {
-  constructor(apiKey: string, timeout: number = 30000) {
+  static readonly DEFAULT_BASE_URL = 'https://api.vultr.com/v2';
+
+  constructor(apiKey: string, baseUrl?: string, timeout: number = 30000) {
     super({
       apiKey,
-      baseUrl: 'https://api.vultr.com/v2',
+      baseUrl: baseUrl || VultrProvider.DEFAULT_BASE_URL,
       timeout,
     });
   }