collab: Add support for subscribing to Zed Pro trials (#28812)

This PR adds support for subscribing to Zed Pro trials (and then
upgrading from a trial to Zed Pro).

Release Notes:

- N/A

crates/collab/src/tests/test_server.rs

@@ -558,6 +558,8 @@ impl TestServer {
                 seed_path: None,
                 stripe_api_key: None,
                 stripe_zed_pro_price_id: None,
+                stripe_zed_pro_trial_price_id: None,
+                stripe_zed_free_price_id: None,
                 supermaven_admin_api_key: None,
                 user_backfiller_github_access_token: None,
                 kinesis_region: None,