Introduce staff-only inline completion provider (#21739)

Release Notes:

- N/A

---------

Co-authored-by: Thorsten Ball <mrnugget@gmail.com>
Co-authored-by: Bennet <bennet@zed.dev>
Co-authored-by: Thorsten <thorsten@zed.dev>

crates/collab/src/tests/test_server.rs

@@ -546,6 +546,9 @@ impl TestServer {
                 anthropic_api_key: None,
                 anthropic_staff_api_key: None,
                 llm_closed_beta_model_name: None,
+                prediction_api_url: None,
+                prediction_api_key: None,
+                prediction_model: None,
                 clickhouse_url: None,
                 clickhouse_user: None,
                 clickhouse_password: None,