Allow customization of the model used for tool calling (#15479)

We also eliminate the `completion` crate and moved its logic into
`LanguageModelRegistry`.

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/collab/src/tests/test_server.rs

@@ -300,7 +300,6 @@ impl TestServer {
             dev_server_projects::init(client.clone(), cx);
             settings::KeymapFile::load_asset(os_keymap, cx).unwrap();
             language_model::LanguageModelRegistry::test(cx);
-            completion::init(cx);
             assistant::context_store::init(&client);
         });