Document prompt cache control behavior

docs/integrations/http-api.md

@@ -128,7 +128,9 @@ Response shape:
     "usage": {
       "prompt_tokens": 11,
       "completion_tokens": 22,
-      "total_tokens": 33
+      "total_tokens": 33,
+      "cached_tokens": 0,
+      "cache_write_tokens": 0
     },
     "start_time": "2026-05-04T12:00:00Z",
     "end_time": "2026-05-04T12:00:01Z",
@@ -142,6 +144,8 @@ Response shape:
 
 `raw_model_output` is omitted by default.
 
+`metadata.usage.cached_tokens` and `metadata.usage.cache_write_tokens` are always present as numbers. They are `0` when the provider omits compatible cache usage fields or reports no cache activity.
+
 To include it, send:
 
 - `"include_raw_output": true`