Introduce the ability to cycle between alternative inline assists (#18098)

Release Notes:

- Added a new `assistant.inline_alternatives` setting to configure
additional models that will be used to perform inline assists in
parallel.

---------

Co-authored-by: Nathan <nathan@zed.dev>
Co-authored-by: Roy <roy@anthropic.com>
Co-authored-by: Adam <wolffiex@anthropic.com>

docs/src/assistant/configuration.md

@@ -20,6 +20,7 @@ To further customize providers, you can use `settings.json` to do that as follow
 - [Configuring endpoints](#custom-endpoint)
 - [Configuring timeouts](#provider-timeout)
 - [Configuring default model](#default-model)
+- [Configuring alternative models for inline assists](#alternative-assists)
 
 ### Zed AI {#zed-ai}
 
@@ -264,6 +265,31 @@ You can also manually edit the `default_model` object in your settings:
 }
 ```
 
+#### Configuring alternative models for inline assists {#alternative-assists}
+
+You can configure additional models that will be used to perform inline assists in parallel. When you do this,
+the inline assist UI will surface controls to cycle between the alternatives generated by each model. The models
+you specify here are always used in _addition_ to your default model. For example, the following configuration
+will generate two outputs for every assist. One with Claude 3.5 Sonnet, and one with GPT-4o.
+
+```json
+{
+  "assistant": {
+    "default_model": {
+      "provider": "zed.dev",
+      "model": "claude-3-5-sonnet"
+    },
+    "inline_alternatives": [
+      {
+        "provider": "zed.dev",
+        "model": "gpt-4o"
+      }
+    ],
+    "version": "2"
+  }
+}
+```
+
 #### Common Panel Settings
 
 | key            | type    | default | description                                                                           |