feat:

### 使用场景 / Use case

自定义供应商 默认只有4个推理模式 然而gpt5.6有6个档位模式。能否支持自定义配置。

### 当前问题 / Current limitation

自定义供应商使用gpt5.6的时候不能使用max推理模式。cindy只有4个固定档位导致。

### 期望方案 / Proposed solution

_No response_

### 已考虑的替代方案 / Alternatives considered

_No response_

Inspect the custom-provider configuration and the existing four-level reasoning-mode selection. Reproduce the limitation with gpt5.6, then determine how reasoning levels can be configured so that the provider exposes all supported modes, including max; done means custom providers are no longer limited to four fixed levels.

Written by the indexing model from the issue text.