collab: Add usage-based billing for LLM interactions (#19081)

This PR adds usage-based billing for LLM interactions in the Assistant.

Release Notes:

- N/A

---------

Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Antonio <antonio@zed.dev>
Co-authored-by: Richard <richard@zed.dev>
Co-authored-by: Richard Feldman <oss@rtfeldman.com>

crates/collab/src/tests/test_server.rs

@@ -635,6 +635,7 @@ impl TestServer {
     ) -> Arc<AppState> {
         Arc::new(AppState {
             db: test_db.db().clone(),
+            llm_db: None,
             live_kit_client: Some(Arc::new(live_kit_test_server.create_api_client())),
             blob_store_client: None,
             stripe_client: None,
@@ -677,8 +678,6 @@ 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,
             },