feat(providers): add Mistral AI provider

Register Mistral as a first-class provider with LiteLLM routing,
MISTRAL_API_KEY env var, and https://api.mistral.ai/v1 default base.

Includes schema field, registry entry, and tests.

nanobot/config/schema.py

@@ -73,6 +73,7 @@ class ProvidersConfig(Base):
     gemini: ProviderConfig = Field(default_factory=ProviderConfig)
     moonshot: ProviderConfig = Field(default_factory=ProviderConfig)
     minimax: ProviderConfig = Field(default_factory=ProviderConfig)
+    mistral: ProviderConfig = Field(default_factory=ProviderConfig)
     aihubmix: ProviderConfig = Field(default_factory=ProviderConfig)  # AiHubMix API gateway
     siliconflow: ProviderConfig = Field(default_factory=ProviderConfig)  # SiliconFlow (硅基流动)
     volcengine: ProviderConfig = Field(default_factory=ProviderConfig)  # VolcEngine (火山引擎)

nanobot/providers/registry.py

@@ -399,6 +399,23 @@ PROVIDERS: tuple[ProviderSpec, ...] = (
         strip_model_prefix=False,
         model_overrides=(),
     ),
+    # Mistral AI: OpenAI-compatible API at api.mistral.ai/v1.
+    ProviderSpec(
+        name="mistral",
+        keywords=("mistral",),
+        env_key="MISTRAL_API_KEY",
+        display_name="Mistral",
+        litellm_prefix="mistral",  # mistral-large-latest → mistral/mistral-large-latest
+        skip_prefixes=("mistral/",),  # avoid double-prefix
+        env_extras=(),
+        is_gateway=False,
+        is_local=False,
+        detect_by_key_prefix="",
+        detect_by_base_keyword="",
+        default_api_base="https://api.mistral.ai/v1",
+        strip_model_prefix=False,
+        model_overrides=(),
+    ),
     # === Local deployment (matched by config key, NOT by api_base) =========
     # vLLM / any OpenAI-compatible local server.
     # Detected when config key is "vllm" (provider_name="vllm").

tests/test_mistral_provider.py

@@ -0,0 +1,22 @@
+"""Tests for the Mistral provider registration."""
+
+from nanobot.config.schema import ProvidersConfig
+from nanobot.providers.registry import PROVIDERS
+
+
+def test_mistral_config_field_exists():
+    """ProvidersConfig should have a mistral field."""
+    config = ProvidersConfig()
+    assert hasattr(config, "mistral")
+
+
+def test_mistral_provider_in_registry():
+    """Mistral should be registered in the provider registry."""
+    specs = {s.name: s for s in PROVIDERS}
+    assert "mistral" in specs
+
+    mistral = specs["mistral"]
+    assert mistral.env_key == "MISTRAL_API_KEY"
+    assert mistral.litellm_prefix == "mistral"
+    assert mistral.default_api_base == "https://api.mistral.ai/v1"
+    assert "mistral/" in mistral.skip_prefixes