fix: include request_headers in MockHTTPClient::clear()

The suggestion to use client->clear() instead of
client->request_headers.items.clear() exposed that clear() only
reset request_body. Update it to also clear request_headers so the
heartbeat session-header test validates fresh headers.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: khanayan123

test/mocks/http_clients.h

@@ -41,7 +41,10 @@ struct MockHTTPClient : public HTTPClient {
   ErrorHandler on_error_;
   std::string request_body;
 
-  void clear() { request_body = ""; }
+  void clear() {
+    request_body = "";
+    request_headers.items.clear();
+  }
 
   Expected<void> post(
       const URL&, HeadersSetter set_headers, std::string body,