docs(providers): explain MiniMax thinking endpoint

Document why MiniMax thinking mode uses a separate Anthropic-compatible provider and list the matching base URLs. Add a small registry test so the new provider stays wired to the expected backend and API key.

Made-with: Cursor

tests/providers/test_minimax_anthropic_provider.py

@@ -0,0 +1,21 @@
+"""Tests for the MiniMax Anthropic provider registration."""
+
+from nanobot.config.schema import ProvidersConfig
+from nanobot.providers.registry import PROVIDERS
+
+
+def test_minimax_anthropic_config_field_exists():
+    """ProvidersConfig should expose a minimax_anthropic field."""
+    config = ProvidersConfig()
+    assert hasattr(config, "minimax_anthropic")
+
+
+def test_minimax_anthropic_provider_in_registry():
+    """MiniMax Anthropic endpoint should be registered with Anthropic backend."""
+    specs = {s.name: s for s in PROVIDERS}
+    assert "minimax_anthropic" in specs
+
+    minimax_anthropic = specs["minimax_anthropic"]
+    assert minimax_anthropic.env_key == "MINIMAX_API_KEY"
+    assert minimax_anthropic.backend == "anthropic"
+    assert minimax_anthropic.default_api_base == "https://api.minimax.io/anthropic"