[AI Gateway] Document Run exhausted-credit errors

src/content/docs/ai-gateway/reference/troubleshooting.mdx

@@ -38,6 +38,14 @@ For troubleshooting Data Loss Prevention issues such as DLP not triggering or un
 
 ## Request failures
 
+### Insufficient credits (`402`)
+
+When a [Run request](/ai-gateway/usage/rest-api/) uses Unified Billing, exhausted credits return HTTP `402` with `internalCode: 2021` in the error response.
+
+[Add credits](/ai-gateway/features/unified-billing/#load-credits) before retrying the request. Retrying without replenishing your balance does not resolve this error.
+
+A background run can also fail because credits are exhausted. An initial `202` response confirms acceptance, not successful completion. If you configured a webhook, check the `state` and `error` fields in its [payload](/ai-gateway/usage/rest-api/#webhook-payload) for the final outcome.
+
 ### Requests timing out
 
 - Check if the upstream provider is experiencing issues

src/content/docs/ai-gateway/usage/rest-api.mdx

@@ -154,7 +154,7 @@ curl -X POST "https://api.cloudflare.com/client/v4/accounts/$CLOUDFLARE_ACCOUNT_
   }'
 ```
 
-A background request returns immediately while the model runs. The result is delivered to your webhook when the run completes.
+An accepted background request returns HTTP `202` while the model runs. This confirms acceptance, not successful completion. AI Gateway sends the final outcome to your webhook when the run finishes.
 
 #### Webhook payload
 
@@ -172,6 +172,8 @@ When the run completes, AI Gateway sends a single `POST` request to your `webhoo
 }
 ```
 
+Check `state` to determine the final outcome. A value of `Completed` indicates success. A value of `Failed` indicates failure, and `error` contains the error message.
+
 Webhook delivery is best-effort and is not retried. The destination must be an HTTPS URL that does not resolve to a private network address.
 
 #### Webhook format