feat: add MiMo (Xiaomi) as AI provider (#887)

* feat: add MiMo (Xiaomi) as AI provider

* fix: correct MiMo default base URL, suggested models, and reasoning support

- Default base URL was the China Token Plan endpoint (tp- keys only);
  switch to https://api.xiaomimimo.com/v1 which works with standard
  pay-as-you-go sk- keys. Token Plan users can override in settings.
- Replace deprecated mimo-v2-flash suggestion with mimo-v2.5
  (v2 series was deprecated on 2026-06-30).
- Use createDeepSeek instead of createOpenAI so reasoning_content is
  passed back during multi-turn tool calls (MiMo returns 400 without
  it), matching the existing Kimi implementation.
- Add mimo to SINGLE_SYSTEM_PROVIDERS so system messages are merged.
- Fold validate-model case into the shared OpenAI-compatible group.
- Drop the Bot icon special case; models.dev serves a real xiaomi logo
  via PROVIDER_LOGO_MAP.
- Document MIMO_API_KEY/MIMO_BASE_URL in env.example and
  docs/{en,cn,ja}/ai-providers.md.

* feat: show base URL hint for MiMo in provider settings

MiMo has two endpoints tied to key type: pay-as-you-go keys (sk-...)
use the default api.xiaomimimo.com/v1, while Token Plan keys (tp-...)
require token-plan-cn.xiaomimimo.com/v1. Surface this under the Base
URL field like the existing MiniMax hint, in all four locales.

---------

Co-authored-by: mapengfei <mapengfei@srsj.com>
Co-authored-by: dayuan.jiang <jdy.toh@gmail.com>

docs/cn/ai-providers.md

@@ -308,6 +308,19 @@ AI_MODEL=your_model_id
 QINIU_BASE_URL=https://your-custom-endpoint
 ```
 
+### MiMo (小米)
+
+```bash
+MIMO_API_KEY=your_api_key
+AI_MODEL=mimo-v2.5-pro
+```
+
+可选的自定义端点（Token Plan 订阅用户请设置专属 Base URL）：
+
+```bash
+MIMO_BASE_URL=https://token-plan-cn.xiaomimimo.com/v1
+```
+
 ## 自动检测
 
 如果您只配置了**一个**提供商的 API 密钥，系统将自动检测并使用该提供商。无需设置 `AI_PROVIDER`。
@@ -315,7 +328,7 @@ QINIU_BASE_URL=https://your-custom-endpoint
 如果您配置了**多个** API 密钥，则必须显式设置 `AI_PROVIDER`：
 
 ```bash
-AI_PROVIDER=google  # 或：openai, anthropic, aihubmix, deepseek, siliconflow, doubao, azure, bedrock, openrouter, ollama, gateway, sglang, modelscope, minimax, glm, qwen, kimi, qiniu
+AI_PROVIDER=google  # 或：openai, anthropic, aihubmix, deepseek, siliconflow, doubao, azure, bedrock, openrouter, ollama, gateway, sglang, modelscope, minimax, glm, qwen, kimi, qiniu, mimo
 ```
 
 ## 服务端多模型配置