Fix a redundant regexp escape in the tool name validation

## Motivation and Context

This PR suppresses the following offense of RuboCop.

```console
$ bundle exec rubocop
Inspecting 163 files
(snip)

Offenses:

lib/mcp/tool.rb:152:48: C: [Correctable] Style/RedundantRegexpEscape: Redundant escape inside regexp literal
        unless tool_name.match?(/\A[A-Za-z\d_\-\.]+\z/)
                                               ^^

163 files inspected, 1 offense detected, 1 offense autocorrectable
```

## How Has This Been Tested?

`bundle exec rake` passes: RuboCop reports no offenses.

## Breaking Changes

None.

Author: koic

lib/mcp/tool.rb

@@ -149,7 +149,7 @@ def validate!
           raise ArgumentError, "Tool names should be between 1 and 128 characters in length (inclusive)."
         end
 
-        unless tool_name.match?(/\A[A-Za-z\d_\-\.]+\z/)
+        unless tool_name.match?(/\A[A-Za-z\d_\-.]+\z/)
           raise ArgumentError, <<~MESSAGE
             Tool names only allowed characters: uppercase and lowercase ASCII letters (A-Z, a-z), digits (0-9), underscore (_), hyphen (-), and dot (.).
           MESSAGE