collab: Fix issues with syncing LLM usage to Stripe (#18970)

This PR fixes some issues with our previous approach to synching LLM
usage over to Stripe.

We now have a separate LLM access price in Stripe that is a marker price
to allow us to create the initial subscription with that as its
subscription item

We then dynamically set the LLM usage price during the reconciliation
sync based on the usage for the current month.

Release Notes:

- N/A

---------

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

crates/collab/src/tests/test_server.rs

@@ -677,6 +677,7 @@ impl TestServer {
                 migrations_path: None,
                 seed_path: None,
                 stripe_api_key: None,
+                stripe_llm_access_price_id: None,
                 stripe_llm_usage_price_id: None,
                 supermaven_admin_api_key: None,
                 user_backfiller_github_access_token: None,