collab: Add LLM request overage columns to billing_preferences (#29446)

This PR adds two new columns to the `billing_preferences` table to allow
users to opt in to overages on LLM requests.

Release Notes:

- N/A

crates/collab/migrations/20250425201930_add_model_request_overages_to_billing_preferences.sql

@@ -0,0 +1,3 @@
+alter table billing_preferences
+    add column model_request_overages_enabled bool not null default false,
+    add column model_request_overages_spend_limit_in_cents integer not null default 0;