language_models: Add reasoning_effort for custom models (#35929)

Release Notes:

- Added `reasoning_effort` support to custom models

Tested using the following config:
```json5
  "language_models": {
    "openai": {
      "available_models": [
        {
          "name": "gpt-5-mini",
          "display_name": "GPT 5 Mini (custom reasoning)",
          "max_output_tokens": 128000,
          "max_tokens": 272000,
          "reasoning_effort": "high" // Can be minimal, low, medium (default), and high
        }
      ],
      "version": "1"
    }
  }
```

Docs:
https://platform.openai.com/docs/api-reference/chat/create#chat_create-reasoning_effort

This work could be used to split the GPT 5/5-mini/5-nano into each of
it's reasoning effort variant. E.g. `gpt-5`, `gpt-5 low`, `gpt-5
minimal`, `gpt-5 high`, and same for mini/nano.

Release Notes:

* Added a setting to control `reasoning_effort` in OpenAI models

crates/language_models/src/provider/open_ai_compatible.rs

@@ -355,7 +355,13 @@ impl LanguageModel for OpenAiCompatibleLanguageModel {
             LanguageModelCompletionError,
         >,
     > {
-        let request = into_open_ai(request, &self.model.name, true, self.max_output_tokens());
+        let request = into_open_ai(
+            request,
+            &self.model.name,
+            true,
+            self.max_output_tokens(),
+            None,
+        );
         let completions = self.stream_completion(request, cx);
         async move {
             let mapper = OpenAiEventMapper::new();