Remove the message copilot_cache_control field from a context where there is no understanding of it (#554)

* Remove copilot_cache_control field from message body when calling endpoints directly. It's a CAPI field and some LLMs will issue an error on this field because it isn't a recognized field.

* remove unneeded conditional expressions

Author: shaunm-msft

src/platform/endpoint/test/node/openaiCompatibleEndpoint.ts

@@ -164,6 +164,14 @@ export class OpenAICompatibleTestEndpoint extends ChatEndpoint {
 			delete body.tools;
 		}
 
+		if (body?.messages) {
+			body.messages.forEach((message: any) => {
+				if (message.copilot_cache_control) {
+					delete message.copilot_cache_control;
+				}
+			});
+		}
+
 		if (body) {
 			if (this.modelConfig.overrides.snippy === null) {
 				delete body.snippy;