collab: Anchor new subscription's billing cycle to the first of the month (#19367)

This PR makes it so new subscriptions will have their billing cycle
anchored to the first of the month.

When someone signs up today, they will be billed starting on the first
of next month.

Release Notes:

- N/A

Co-authored-by: Antonio <antontio@zed.dev>
Co-authored-by: Richard <richard@zed.dev>

crates/collab/src/stripe_billing.rs

@@ -2,7 +2,7 @@ use std::sync::Arc;
 
 use crate::{llm, Cents, Result};
 use anyhow::Context;
-use chrono::Utc;
+use chrono::{Datelike, Utc};
 use collections::HashMap;
 use serde::{Deserialize, Serialize};
 use tokio::sync::RwLock;
@@ -349,10 +349,20 @@ impl StripeBilling {
         model: &StripeModel,
         success_url: &str,
     ) -> Result<String> {
+        let first_of_next_month = Utc::now()
+            .checked_add_months(chrono::Months::new(1))
+            .unwrap()
+            .with_day(1)
+            .unwrap();
+
         let mut params = stripe::CreateCheckoutSession::new();
         params.mode = Some(stripe::CheckoutSessionMode::Subscription);
         params.customer = Some(customer_id);
         params.client_reference_id = Some(github_login);
+        params.subscription_data = Some(stripe::CreateCheckoutSessionSubscriptionData {
+            billing_cycle_anchor: Some(first_of_next_month.timestamp()),
+            ..Default::default()
+        });
         params.line_items = Some(
             [
                 &model.input_tokens_price.id,