fix: use Anthropic SDK provider for anthropic protocol with API key

The anthropic protocol case in CreateProviderFromConfig incorrectly used
NewHTTPProviderWithMaxTokensFieldAndRequestTimeout (OpenAI-compatible
HTTP provider) when an API key was provided. This sent OpenAI-format
requests to Anthropic-compatible endpoints, causing 404 errors.

Switch to NewClaudeProviderWithBaseURL which uses the official Anthropic
SDK and sends proper Anthropic Messages API format requests. This fixes
model_list entries like:

  {"model": "anthropic/glm-4.7", "api_base": "https://api.z.ai/api/anthropic/v1"}

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/providers/factory_provider.go

@@ -120,21 +120,11 @@ func CreateProviderFromConfig(cfg *config.ModelConfig) (LLMProvider, string, err
 			}
 			return provider, modelID, nil
 		}
-		// Use API key with HTTP API
-		apiBase := cfg.APIBase
-		if apiBase == "" {
-			apiBase = "https://api.anthropic.com/v1"
-		}
+		// Use API key with Anthropic Messages API
 		if cfg.APIKey == "" {
 			return nil, "", fmt.Errorf("api_key is required for anthropic protocol (model: %s)", cfg.Model)
 		}
-		return NewHTTPProviderWithMaxTokensFieldAndRequestTimeout(
-			cfg.APIKey,
-			apiBase,
-			cfg.Proxy,
-			cfg.MaxTokensField,
-			cfg.RequestTimeout,
-		), modelID, nil
+		return NewClaudeProviderWithBaseURL(cfg.APIKey, cfg.APIBase), modelID, nil
 
 	case "antigravity":
 		return NewAntigravityProvider(), modelID, nil