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/linode-provider.ts

@@ -26,10 +26,12 @@ interface LinodeError {
 }
 
 export class LinodeProvider extends VPSProviderBase {
-  constructor(apiKey: string, timeout: number = 30000) {
+  static readonly DEFAULT_BASE_URL = 'https://api.linode.com/v4';
+
+  constructor(apiKey: string, baseUrl?: string, timeout: number = 30000) {
     super({
       apiKey,
-      baseUrl: 'https://api.linode.com/v4',
+      baseUrl: baseUrl || LinodeProvider.DEFAULT_BASE_URL,
       timeout,
     });
   }
@@ -58,18 +60,32 @@ export class LinodeProvider extends VPSProviderBase {
         body: JSON.stringify(body),
       });
 
+      // Read response text first to handle both JSON and HTML errors
+      const responseText = await response.text();
+
       if (!response.ok) {
-        const error = (await response.json()) as LinodeError;
-        return {
-          success: false,
-          error: {
-            code: `LINODE_${response.status}`,
-            message: error.errors?.[0]?.reason || 'Unknown error',
-          },
-        };
+        // Try to parse as JSON, but handle HTML responses gracefully
+        try {
+          const error = JSON.parse(responseText) as LinodeError;
+          return {
+            success: false,
+            error: {
+              code: `LINODE_${response.status}`,
+              message: error.errors?.[0]?.reason || 'Unknown error',
+            },
+          };
+        } catch {
+          return {
+            success: false,
+            error: {
+              code: `LINODE_${response.status}`,
+              message: `Non-JSON response: ${responseText.substring(0, 200)}`,
+            },
+          };
+        }
       }
 
-      const data = (await response.json()) as LinodeInstance;
+      const data = JSON.parse(responseText) as LinodeInstance;
 
       return {
         success: true,