agent: Fix test

Struct TtrpcContext has been changed in ttrpc@0.5

Signed-off-by: Tim Zhang <tim@hyper.sh>

src/agent/src/rpc.rs

@@ -1676,6 +1676,7 @@ mod tests {
         TtrpcContext {
             fd: -1,
             mh: MessageHeader::default(),
+            metadata: std::collections::HashMap::new(),
         }
     }