chore: add env var override for the SendMessage API, for testing purposes (#843)

Author: brandonskiser

crates/fig_api_client/src/clients/streaming_client.rs

@@ -52,7 +52,9 @@ pub struct StreamingClient(inner::Inner);
 
 impl StreamingClient {
     pub async fn new() -> Result<Self, Error> {
-        let client = if fig_util::system_info::in_cloudshell() {
+        let client = if fig_util::system_info::in_cloudshell()
+            || std::env::var("Q_USE_SENDMESSAGE").is_ok_and(|v| !v.is_empty())
+        {
             Self::new_qdeveloper_client(&Endpoint::load_q()).await?
         } else {
             Self::new_codewhisperer_client(&Endpoint::load_codewhisperer()).await