docs: add Xiaomi MiMo to all provider docs + fix MiMo-V2-Flash ctx len

- environment-variables.md: XIAOMI_API_KEY, XIAOMI_BASE_URL, provider list
- cli-commands.md: --provider choices
- integrations/providers.md: provider table, Chinese providers section,
  config example, base URL list, choosing table, fallback providers list
- fallback-providers.md: supported providers table, auto-detection chain
- Fix XiaomiMiMo/MiMo-V2-Flash context length 32768 → 256000 (OpenRouter entry)

agent/auxiliary_client.py

@@ -109,7 +109,6 @@ _API_KEY_PROVIDER_AUX_MODELS: Dict[str, str] = {
     "opencode-zen": "gemini-3-flash",
     "opencode-go": "glm-5",
     "kilocode": "google/gemini-3-flash-preview",
-    "xiaomi": "mimo-v2-flash",
 }
 
 # Vision-specific model overrides for direct providers.

agent/model_metadata.py

@@ -151,7 +151,7 @@ DEFAULT_CONTEXT_LENGTHS = {
     "moonshotai/Kimi-K2.5": 262144,
     "moonshotai/Kimi-K2-Thinking": 262144,
     "MiniMaxAI/MiniMax-M2.5": 204800,
-    "XiaomiMiMo/MiMo-V2-Flash": 32768,
+    "XiaomiMiMo/MiMo-V2-Flash": 256000,
     "mimo-v2-pro": 1000000,
     "mimo-v2-omni": 256000,
     "mimo-v2-flash": 256000,