feat(xiaomi): add Xiaomi MiMo as first-class provider

Cherry-picked from PR #7702 by kshitijk4poor.

Adds Xiaomi MiMo as a direct provider (XIAOMI_API_KEY) with models:
- mimo-v2-pro (1M context), mimo-v2-omni (256K, multimodal), mimo-v2-flash (256K, cheapest)

Standard OpenAI-compatible provider checklist: auth.py, config.py, models.py,
main.py, providers.py, doctor.py, model_normalize.py, model_metadata.py,
models_dev.py, auxiliary_client.py, .env.example, cli-config.yaml.example.

Follow-up: vision tasks use mimo-v2-omni (multimodal) instead of the user's
main model. Non-vision aux uses the user's selected model. Added
_PROVIDER_VISION_MODELS dict for provider-specific vision model overrides.
On failure, falls back to aggregators (gemini flash) via existing fallback chain.

Corrects pre-existing context lengths: mimo-v2-pro 1048576→1000000,
mimo-v2-omni 1048576→256000, adds mimo-v2-flash 256000.

36 tests covering registry, aliases, auto-detect, credentials, models.dev,
normalization, URL mapping, providers module, doctor, aux client, vision
model override, and agent init.

agent/model_metadata.py

@@ -27,12 +27,14 @@ _PROVIDER_PREFIXES: frozenset[str] = frozenset({
     "gemini", "zai", "kimi-coding", "minimax", "minimax-cn", "anthropic", "deepseek",
     "opencode-zen", "opencode-go", "ai-gateway", "kilocode", "alibaba",
     "qwen-oauth",
+    "xiaomi",
     "custom", "local",
     # Common aliases
     "google", "google-gemini", "google-ai-studio",
     "glm", "z-ai", "z.ai", "zhipu", "github", "github-copilot",
     "github-models", "kimi", "moonshot", "claude", "deep-seek",
     "opencode", "zen", "go", "vercel", "kilo", "dashscope", "aliyun", "qwen",
+    "mimo", "xiaomi-mimo",
     "qwen-portal",
 })
 
@@ -150,8 +152,9 @@ DEFAULT_CONTEXT_LENGTHS = {
     "moonshotai/Kimi-K2-Thinking": 262144,
     "MiniMaxAI/MiniMax-M2.5": 204800,
     "XiaomiMiMo/MiMo-V2-Flash": 32768,
-    "mimo-v2-pro": 1048576,
-    "mimo-v2-omni": 1048576,
+    "mimo-v2-pro": 1000000,
+    "mimo-v2-omni": 256000,
+    "mimo-v2-flash": 256000,
     "zai-org/GLM-5": 202752,
 }
 
@@ -211,6 +214,8 @@ _URL_TO_PROVIDER: Dict[str, str] = {
     "api.fireworks.ai": "fireworks",
     "opencode.ai": "opencode-go",
     "api.x.ai": "xai",
+    "api.xiaomimimo.com": "xiaomi",
+    "xiaomimimo.com": "xiaomi",
 }