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/cloud.rs

@@ -942,6 +942,7 @@ impl LanguageModel for CloudLanguageModel {
                     model.id(),
                     model.supports_parallel_tool_calls(),
                     None,
+                    None,
                 );
                 let llm_api_token = self.llm_api_token.clone();
                 let future = self.request_limiter.stream(async move {