collab: Add ability to initiate a checkout session for the Zed Free plan (#29767)

This PR adds the ability to initiate a checkout session for the Zed Free
plan.

Release Notes:

- N/A

crates/collab/src/api/billing.rs

@@ -256,6 +256,7 @@ async fn list_billing_subscriptions(
 enum ProductCode {
     ZedPro,
     ZedProTrial,
+    ZedFree,
 }
 
 #[derive(Debug, Deserialize)]
@@ -386,6 +387,11 @@ async fn create_billing_subscription(
                 )
                 .await?
         }
+        Some(ProductCode::ZedFree) => {
+            stripe_billing
+                .checkout_with_zed_free(customer_id, &user.github_login, &success_url)
+                .await?
+        }
         None => {
             let default_model = llm_db.model(
                 zed_llm_client::LanguageModelProvider::Anthropic,