collab: Transfer existing usage from trial to Pro (#28884)

This PR adds support for transferring any existing usage from a trial
subscription to a Zed Pro subscription when the user upgrades.

Release Notes:

- N/A

---------

Co-authored-by: Mikayla <mikayla@zed.dev>

crates/collab/src/llm/db/tests/subscription_usage_tests.rs

@@ -0,0 +1,69 @@
+use chrono::{Duration, Utc};
+use pretty_assertions::assert_eq;
+
+use crate::db::billing_subscription::SubscriptionKind;
+use crate::db::{UserId, billing_subscription};
+use crate::llm::db::LlmDatabase;
+use crate::test_llm_db;
+
+test_llm_db!(
+    test_transfer_existing_subscription_usage,
+    test_transfer_existing_subscription_usage_postgres
+);
+
+async fn test_transfer_existing_subscription_usage(db: &mut LlmDatabase) {
+    let user_id = UserId(1);
+
+    let now = Utc::now();
+
+    let trial_period_start_at = now - Duration::days(14);
+    let trial_period_end_at = now;
+
+    let new_period_start_at = now;
+    let new_period_end_at = now + Duration::days(30);
+
+    let existing_subscription = billing_subscription::Model {
+        kind: Some(SubscriptionKind::ZedProTrial),
+        stripe_current_period_start: Some(trial_period_start_at.timestamp()),
+        stripe_current_period_end: Some(trial_period_end_at.timestamp()),
+        ..Default::default()
+    };
+
+    let existing_usage = db
+        .create_subscription_usage(
+            user_id,
+            trial_period_start_at,
+            trial_period_end_at,
+            SubscriptionKind::ZedProTrial,
+            25,
+            1_000,
+        )
+        .await
+        .unwrap();
+
+    let transferred_usage = db
+        .transfer_existing_subscription_usage(
+            user_id,
+            &existing_subscription,
+            Some(SubscriptionKind::ZedPro),
+            new_period_start_at,
+            new_period_end_at,
+        )
+        .await
+        .unwrap();
+
+    assert!(
+        transferred_usage.is_some(),
+        "subscription usage not transferred successfully"
+    );
+    let transferred_usage = transferred_usage.unwrap();
+
+    assert_eq!(
+        transferred_usage.model_requests,
+        existing_usage.model_requests
+    );
+    assert_eq!(
+        transferred_usage.edit_predictions,
+        existing_usage.edit_predictions
+    );
+}