collab: Fully move StripeBilling over to using StripeClient (#31722)

This PR moves over the last method on `StripeBilling` to use the
`StripeClient` trait, allowing us to fully mock out Stripe behaviors for
`StripeBilling` in tests.

Release Notes:

- N/A

crates/collab/src/rpc.rs

@@ -5,6 +5,7 @@ use crate::api::{CloudflareIpCountryHeader, SystemIdHeader};
 use crate::db::billing_subscription::SubscriptionKind;
 use crate::llm::db::LlmDatabase;
 use crate::llm::{AGENT_EXTENDED_TRIAL_FEATURE_FLAG, LlmTokenClaims};
+use crate::stripe_client::StripeCustomerId;
 use crate::{
     AppState, Error, Result, auth,
     db::{
@@ -4055,10 +4056,8 @@ async fn get_llm_api_token(
         if let Some(billing_subscription) = db.get_active_billing_subscription(user.id).await? {
             billing_subscription
         } else {
-            let stripe_customer_id = billing_customer
-                .stripe_customer_id
-                .parse::<stripe::CustomerId>()
-                .context("failed to parse Stripe customer ID from database")?;
+            let stripe_customer_id =
+                StripeCustomerId(billing_customer.stripe_customer_id.clone().into());
 
             let stripe_subscription = stripe_billing
                 .subscribe_to_zed_free(stripe_customer_id)