collab: Only subscribe to Zed Free if there isn't an existing active subscription (#30967)

This PR adds a sanity check to ensure that we only subscribe the user to
Zed Free if they don't already have an active subscription.

Release Notes:

- N/A

crates/collab/src/api/billing.rs

@@ -1183,6 +1183,11 @@ async fn sync_subscription(
         if subscription.status == SubscriptionStatus::Canceled
             || subscription.status == SubscriptionStatus::Paused
         {
+            let already_has_active_billing_subscription = app
+                .db
+                .has_active_billing_subscription(billing_customer.user_id)
+                .await?;
+            if !already_has_active_billing_subscription {
                 let stripe_customer_id = billing_customer
                     .stripe_customer_id
                     .parse::<stripe::CustomerId>()
@@ -1193,6 +1198,7 @@ async fn sync_subscription(
                     .await?;
             }
         }
+    }
 
     Ok(billing_customer)
 }