fix: add better error handling for uninitialized billing plugin

- Fix TypeError when accessing providerConfig on undefined billing plugin
- Add proper type safety: useBillingPlugin now returns BillingPlugin | undefined
- Add clear error message when plugin hasn't been initialized
- Update README quickstart with concise provider response examples

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/plugin/index.ts

@@ -13,7 +13,7 @@ type BillingPlugin = {
   }
 }
 
-export const useBillingPlugin = (payload: Payload) => singleton.get(payload) as BillingPlugin
+export const useBillingPlugin = (payload: Payload) => singleton.get(payload) as BillingPlugin | undefined
 
 export const billingPlugin = (pluginConfig: BillingPluginConfig = {}) => (config: Config): Config => {
   if (pluginConfig.disabled) {