refactor: cleanup provider base class; cleanup shared responsibilities; cleanup public contract

docs: document provider base class
refactor: cleanup custom provider, it should only deal with `is_custom` model configurations
fix: make sure openrouter provider does not load `is_custom` models
fix: listmodels tool cleanup

tests/test_xai_provider.py

@@ -141,7 +141,7 @@ class TestXAIProvider:
         """Test error handling for unsupported models."""
         provider = XAIModelProvider("test-key")
 
-        with pytest.raises(ValueError, match="Unsupported X.AI model"):
+        with pytest.raises(ValueError, match="Unsupported model 'invalid-model' for provider xai"):
             provider.get_capabilities("invalid-model")
 
     def test_extended_thinking_flags(self):