feat: antropik cache — default anthropic-version, forward cache headers, model features, streaming usage, tests

- Auto-default anthropic-version to 2023-06-01 for prompt caching
- Forward cache headers (x-cache, cf-cache-status, age, etc.) from backend
- Add features: ["prompt_caching"] to /v1/models response
- Extract and report actual usage tokens in streaming message_delta
- Track output char count for token estimation when backend omits it
- Cache-aware usage reporting: cache_creation_input_tokens, cache_read_input_tokens
- Comprehensive tests for cache_control preservation in content blocks & system

src/worker.ts

@@ -471,6 +471,7 @@ export default {
 				object: "model",
 				created: Math.floor(Date.now() / 1000),
 				owned_by: "proxy",
+				features: ["prompt_caching"],
 			}));
 			return new Response(
 				JSON.stringify({ object: "list", data: models }),