collab: Add billing thresholds to request overage subscription items (#29738)

This PR adds billing thresholds of the unit equivalent of $20 for model
request overages.

Release Notes:

- N/A

crates/collab/src/stripe_billing.rs

@@ -99,6 +99,16 @@ impl StripeBilling {
             .ok_or_else(|| crate::Error::Internal(anyhow!("no price ID found for {lookup_key:?}")))
     }
 
+    pub async fn find_price_by_lookup_key(&self, lookup_key: &str) -> Result<stripe::Price> {
+        self.state
+            .read()
+            .await
+            .prices_by_lookup_key
+            .get(lookup_key)
+            .cloned()
+            .ok_or_else(|| crate::Error::Internal(anyhow!("no price found for {lookup_key:?}")))
+    }
+
     pub async fn register_model_for_token_based_usage(
         &self,
         model: &llm::db::model::Model,
@@ -238,21 +248,29 @@ impl StripeBilling {
     pub async fn subscribe_to_price(
         &self,
         subscription_id: &stripe::SubscriptionId,
-        price_id: &stripe::PriceId,
+        price: &stripe::Price,
     ) -> Result<()> {
         let subscription =
             stripe::Subscription::retrieve(&self.client, &subscription_id, &[]).await?;
 
-        if subscription_contains_price(&subscription, price_id) {
+        if subscription_contains_price(&subscription, &price.id) {
             return Ok(());
         }
 
+        const BILLING_THRESHOLD_IN_CENTS: i64 = 20 * 100;
+
+        let price_per_unit = price.unit_amount.unwrap_or_default();
+        let units_for_billing_threshold = BILLING_THRESHOLD_IN_CENTS / price_per_unit;
+
         stripe::Subscription::update(
             &self.client,
             subscription_id,
             stripe::UpdateSubscription {
                 items: Some(vec![stripe::UpdateSubscriptionItems {
-                    price: Some(price_id.to_string()),
+                    price: Some(price.id.to_string()),
+                    billing_thresholds: Some(stripe::SubscriptionItemBillingThresholds {
+                        usage_gte: Some(units_for_billing_threshold),
+                    }),
                     ..Default::default()
                 }]),
                 trial_settings: Some(stripe::UpdateSubscriptionTrialSettings {