Add a default_open_ai_model setting for the assistant (#2876)

[This PR has been sitting around for a
bit](https://github.com/zed-industries/zed/pull/2845). I received a bit
of mixed opinions from the team on how this setting should work, if it
should use the full model names or some simpler form of it, etc. I went
ahead and made the decision to do the following:

- Use the full model names in settings - ex: `gpt-4-0613`
- Default to `gpt-4-0613` when no setting is present
- Save the full model names in the conversation history files (this is
how it was prior) - ex: `gpt-4-0613`
- Display the shortened model names in the assistant - ex: `gpt-4`
- Not worry about adding an option to add custom models (can add in a
follow-up PR)
- Not query what models are available to the user via their api key (can
add in a follow-up PR)

Release Notes:

- Added a `default_open_ai_model` setting for the assistant (defaults to
`gpt-4-0613`).

---------

Co-authored-by: Mikayla <mikayla@zed.dev>

crates/settings/src/settings_store.rs

@@ -1,4 +1,4 @@
-use anyhow::{anyhow, Result};
+use anyhow::{anyhow, Context, Result};
 use collections::{btree_map, hash_map, BTreeMap, HashMap};
 use gpui::AppContext;
 use lazy_static::lazy_static;
@@ -162,6 +162,7 @@ impl SettingsStore {
 
             if let Some(setting) = setting_value
                 .load_setting(&default_settings, &user_values_stack, cx)
+                .context("A default setting must be added to the `default.json` file")
                 .log_err()
             {
                 setting_value.set_global_value(setting);