feat: anthropic cache support — preserve cache_control, forward anthropic-version header, report real usage tokens

- Add anthropicPassthrough flag to BackendConfig for native Anthropic backends
- Preserve cache_control on content blocks and system prompt (keep structured)
- Extract and forward anthropic-version header from client to backend
- Report actual token usage from backend response (input_tokens, output_tokens)
- Support native Anthropic passthrough (no translation) for compatible backends
- Wire anthropic-version through all entry points: index.ts, router.ts, worker.ts

src/worker.ts

@@ -453,7 +453,8 @@ export default {
 			if (authErr) return authErr;
 			try {
 				const body = await req.json();
-				return handleAnthropicMessages(body);
+				const anthropicVersion = req.headers.get("anthropic-version") ?? undefined;
+				return handleAnthropicMessages(body, undefined, undefined, undefined, undefined, anthropicVersion);
 			} catch {
 				return new Response(
 					JSON.stringify({ error: { message: "Invalid JSON body", type: "invalid_request_error" } }),