language_models: Add thinking support to OpenRouter provider (#32541)

Did some bit cleanup of code for loading models for settings as that is
not required as we are fetching all the models from openrouter so it's
better to maintain one source of truth

Release Notes:

- Add thinking support to OpenRouter provider

docs/src/ai/configuration.md

@@ -489,6 +489,49 @@ The OpenRouter API key will be saved in your keychain.
 
 Zed will also use the `OPENROUTER_API_KEY` environment variable if it's defined.
 
+#### Custom Models {#openrouter-custom-models}
+
+You can add custom models to the OpenRouter provider by adding the following to your Zed `settings.json`:
+
+```json
+{
+  "language_models": {
+    "open_router": {
+      "api_url": "https://openrouter.ai/api/v1",
+      "available_models": [
+        {
+          "name": "google/gemini-2.0-flash-thinking-exp",
+          "display_name": "Gemini 2.0 Flash (Thinking)",
+          "max_tokens": 200000,
+          "max_output_tokens": 8192,
+          "supports_tools": true,
+          "supports_images": true,
+          "mode": {
+            "type": "thinking",
+            "budget_tokens": 8000
+          }
+        }
+      ]
+    }
+  }
+}
+```
+
+The available configuration options for each model are:
+
+- `name`: The model identifier used by OpenRouter (required)
+- `display_name`: A human-readable name shown in the UI (optional)
+- `max_tokens`: The model's context window size (required)
+- `max_output_tokens`: Maximum tokens the model can generate (optional)
+- `max_completion_tokens`: Maximum completion tokens (optional)
+- `supports_tools`: Whether the model supports tool/function calling (optional)
+- `supports_images`: Whether the model supports image inputs (optional)
+- `mode`: Special mode configuration for thinking models (optional)
+
+You can find available models and their specifications on the [OpenRouter models page](https://openrouter.ai/models).
+
+Custom models will be listed in the model dropdown in the Agent Panel.
+
 ## Advanced Configuration {#advanced-configuration}
 
 ### Custom Provider Endpoints {#custom-provider-endpoint}