Fall back to StructuredContent in MCP tool results

When an MCP server sets StructuredContent without populating
Content with TextContent entries, extractMCPContent returned
an empty string, making successful tool calls look empty to
the agent. Now the function serializes StructuredContent as
JSON when no text parts are found.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/agent/mcp.go

@@ -167,7 +167,7 @@ func (t *mcpTool) Execute(ctx context.Context, arguments string) (ToolResult, er
 }
 
 func extractMCPContent(result *mcp.CallToolResult) string {
-	if result == nil || len(result.Content) == 0 {
+	if result == nil {
 		return ""
 	}
 
@@ -178,5 +178,16 @@ func extractMCPContent(result *mcp.CallToolResult) string {
 		}
 	}
 
-	return strings.Join(parts, "\n")
+	if len(parts) > 0 {
+		return strings.Join(parts, "\n")
+	}
+
+	if result.StructuredContent != nil {
+		data, err := json.Marshal(result.StructuredContent)
+		if err == nil {
+			return string(data)
+		}
+	}
+
+	return ""
 }