collab: Use StripeClient in sync_subscription (#31761)

This PR updates the `sync_subscription` function to use the
`StripeClient` trait instead of using `stripe::Client` directly.

Release Notes:

- N/A

---------

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

crates/collab/src/tests/test_server.rs

@@ -1,3 +1,4 @@
+use crate::stripe_client::FakeStripeClient;
 use crate::{
     AppState, Config,
     db::{NewUserParams, UserId, tests::TestDb},
@@ -522,7 +523,8 @@ impl TestServer {
             llm_db: None,
             livekit_client: Some(Arc::new(livekit_test_server.create_api_client())),
             blob_store_client: None,
-            stripe_client: None,
+            real_stripe_client: None,
+            stripe_client: Some(Arc::new(FakeStripeClient::new())),
             stripe_billing: None,
             executor,
             kinesis_client: None,