[AI Gateway] Mention Unified API alongside provider-native endpoints for HTTP API billing

Author: ethulia

src/content/docs/ai-gateway/features/unified-billing.mdx

@@ -90,7 +90,7 @@ curl -X POST https://gateway.ai.cloudflare.com/v1/$CLOUDFLARE_ACCOUNT_ID/default
 
 The `default` gateway is created automatically on your first request. Replace `default` with a specific gateway ID if you have already created one.
 
-The HTTP API supports the following providers through their [provider-native endpoints](/ai-gateway/usage/providers/):
+The HTTP API supports the following providers through [provider-native endpoints](/ai-gateway/usage/providers/) and the [Unified API (chat completions)](/ai-gateway/usage/chat-completion/):
 
 - [OpenAI](/ai-gateway/usage/providers/openai/)
 - [Anthropic](/ai-gateway/usage/providers/anthropic/)