fix: put anthropic_version in request body, not header

SAP AI Core expects anthropic_version in the JSON body, not as a header.

llm/aicore.go

@@ -253,6 +253,7 @@ func (c *AICoreClient) CompleteAnthropic(ctx context.Context, model string, mess
 	}
 
 	reqBody := anthropicRequest{
+		AnthropicVersion: "2023-06-01",
 		Model:     model,
 		MaxTokens: maxTokens,
 		System:    system,
@@ -276,7 +277,6 @@ func (c *AICoreClient) CompleteAnthropic(ctx context.Context, model string, mess
 	req.Header.Set("Authorization", "Bearer "+token)
 	req.Header.Set("AI-Resource-Group", c.config.ResourceGroup)
 	req.Header.Set("Content-Type", "application/json")
-	req.Header.Set("anthropic-version", "2023-06-01")
 
 	resp, err := c.http.Do(req)
 	if err != nil {

llm/client.go

@@ -206,6 +206,7 @@ func (c *Client) completeOpenAI(ctx context.Context, messages []Message) (string
 // --- Anthropic Messages API implementation ---
 
 type anthropicRequest struct {
+	AnthropicVersion string         `json:"anthropic_version,omitempty"`
 	Model       string         `json:"model"`
 	MaxTokens   int            `json:"max_tokens"`
 	System      string         `json:"system,omitempty"`