collab: Add Zed Pro checkout flow (#28776)

This PR adds support for initiating a checkout flow for Zed Pro.

Release Notes:

- N/A

crates/collab/src/tests/test_server.rs

@@ -557,6 +557,7 @@ impl TestServer {
                 migrations_path: None,
                 seed_path: None,
                 stripe_api_key: None,
+                stripe_zed_pro_price_id: None,
                 supermaven_admin_api_key: None,
                 user_backfiller_github_access_token: None,
                 kinesis_region: None,