fix(anthropic): pass through top-level cache_control for automatic caching

Anthropic supports automatic prompt caching via a top-level cache_control
field on the request body. This was not being forwarded to the provider.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: bhuvansingla

src/providers/anthropic/chatComplete.ts

@@ -469,6 +469,10 @@ export const AnthropicChatCompleteConfig: ProviderConfig = {
     param: 'thinking',
     required: false,
   },
+  cache_control: {
+    param: 'cache_control',
+    required: false,
+  },
 };
 
 interface AnthorpicTextContentItem {