Refactor to extract duplicate error tool response in MCP::Server

Consolidates four identical constructions of error `Tool::Response` in
`handle_call_tool_request` into a single private `error_tool_response` method,
reducing duplication and improving maintainability. No behavior change.

Author: koic

lib/mcp/server.rb

@@ -262,56 +262,36 @@ def call_tool(request)
       tool = tools[tool_name]
       unless tool
         add_instrumentation_data(tool_name:, error: :tool_not_found)
-        return Tool::Response.new(
-          [{
-            type: "text",
-            text: "Tool not found: #{tool_name}",
-          }],
-          error: true,
-        ).to_h
+
+        return error_tool_response("Tool not found: #{tool_name}")
       end
 
       arguments = request[:arguments] || {}
       add_instrumentation_data(tool_name:)
 
       if tool.input_schema&.missing_required_arguments?(arguments)
         add_instrumentation_data(error: :missing_required_arguments)
+
         missing = tool.input_schema.missing_required_arguments(arguments).join(", ")
-        return Tool::Response.new(
-          [{
-            type: "text",
-            text: "Missing required arguments: #{missing}",
-          }],
-          error: true,
-        ).to_h
+        return error_tool_response("Missing required arguments: #{missing}")
       end
 
       if configuration.validate_tool_call_arguments && tool.input_schema
         begin
           tool.input_schema.validate_arguments(arguments)
         rescue Tool::InputSchema::ValidationError => e
           add_instrumentation_data(error: :invalid_schema)
-          return Tool::Response.new(
-            [{
-              type: "text",
-              text: e.message,
-            }],
-            error: true,
-          ).to_h
+
+          return error_tool_response(e.message)
         end
       end
 
       begin
         call_tool_with_args(tool, arguments)
       rescue => e
         report_exception(e, { request: request })
-        Tool::Response.new(
-          [{
-            type: "text",
-            text: "Internal error calling tool #{tool_name}: #{e.message}",
-          }],
-          error: true,
-        ).to_h
+
+        error_tool_response("Internal error calling tool #{tool_name}: #{e.message}")
       end
     end
 
@@ -359,6 +339,16 @@ def index_resources_by_uri(resources)
       end
     end
 
+    def error_tool_response(text)
+      Tool::Response.new(
+        [{
+          type: "text",
+          text: text,
+        }],
+        error: true,
+      ).to_h
+    end
+
     def accepts_server_context?(method_object)
       parameters = method_object.parameters