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.

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