collab: Prevent canceling the free plan (#31292)

This PR makes it so the Zed Free plan cannot be canceled.

We were already preventing this on the zed.dev side, but this will make
it more airtight.

Release Notes:

- N/A

crates/collab/src/api/billing.rs

@@ -269,7 +269,8 @@ async fn list_billing_subscriptions(
                         .and_utc()
                         .to_rfc3339_opts(SecondsFormat::Millis, true)
                 }),
-                is_cancelable: subscription.stripe_subscription_status.is_cancelable()
+                is_cancelable: subscription.kind != Some(SubscriptionKind::ZedFree)
+                    && subscription.stripe_subscription_status.is_cancelable()
                     && subscription.stripe_cancel_at.is_none(),
             })
             .collect(),
@@ -591,7 +592,15 @@ async fn manage_billing_subscription(
             }),
             ..Default::default()
         }),
-        ManageSubscriptionIntent::Cancel => Some(CreateBillingPortalSessionFlowData {
+        ManageSubscriptionIntent::Cancel => {
+            if subscription.kind == Some(SubscriptionKind::ZedFree) {
+                return Err(Error::http(
+                    StatusCode::BAD_REQUEST,
+                    "free subscription cannot be canceled".into(),
+                ));
+            }
+
+            Some(CreateBillingPortalSessionFlowData {
                 type_: CreateBillingPortalSessionFlowDataType::SubscriptionCancel,
                 after_completion: Some(CreateBillingPortalSessionFlowDataAfterCompletion {
                     type_: stripe::CreateBillingPortalSessionFlowDataAfterCompletionType::Redirect,
@@ -607,7 +616,8 @@ async fn manage_billing_subscription(
                     },
                 ),
                 ..Default::default()
-        }),
+            })
+        }
         ManageSubscriptionIntent::StopCancellation => unreachable!(),
     };