Fix tool error handling to follow MCP spec

Tool execution errors now return successful responses with isError: true
instead of JSON-RPC protocol errors, per the MCP specification.

This allows clients to distinguish between protocol-level errors and
tool execution errors, providing better error context.

Fixes #159

lib/mcp/server.rb

@@ -258,10 +258,17 @@ module MCP
 
     def call_tool(request)
       tool_name = request[:name]
+
       tool = tools[tool_name]
       unless tool
-        add_instrumentation_data(error: :tool_not_found)
-        raise RequestHandlerError.new("Tool not found #{tool_name}", request, error_type: :tool_not_found)
+        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
       end
 
       arguments = request[:arguments] || {}
@@ -269,11 +276,14 @@ module MCP
 
       if tool.input_schema&.missing_required_arguments?(arguments)
         add_instrumentation_data(error: :missing_required_arguments)
-        raise RequestHandlerError.new(
-          "Missing required arguments: #{tool.input_schema.missing_required_arguments(arguments).join(", ")}",
-          request,
-          error_type: :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
       end
 
       if configuration.validate_tool_call_arguments && tool.input_schema
@@ -281,14 +291,27 @@ module MCP
           tool.input_schema.validate_arguments(arguments)
         rescue Tool::InputSchema::ValidationError => e
           add_instrumentation_data(error: :invalid_schema)
-          raise RequestHandlerError.new(e.message, request, error_type: :invalid_schema)
+          return Tool::Response.new(
+            [{
+              type: "text",
+              text: e.message,
+            }],
+            error: true,
+          ).to_h
         end
       end
 
       begin
         call_tool_with_args(tool, arguments)
       rescue => e
-        raise RequestHandlerError.new("Internal error calling tool #{tool_name}", request, original_error: e)
+        report_exception(e, { request: request })
+        Tool::Response.new(
+          [{
+            type: "text",
+            text: "Internal error calling tool #{tool_name}: #{e.message}",
+          }],
+          error: true,
+        ).to_h
       end
     end