Fix ReDoS vulnerability in StringUtils#underscore

## Summary

Replace regex patterns with lookahead/lookbehind assertions to prevent
catastrophic backtracking on Ruby 3.1 and earlier.

This issue only affects Ruby 3.1 and earlier. Since it does not occur on Ruby 3.2+,
it is not an issue in the current stable release (0.4.0), which does not include #206.

## Regression Testing

Ensure the regex patterns complete within 1 second for a 50,000-character input
that would otherwise trigger superlinear backtracking.

test/mcp/string_utils_test.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "test_helper"
+require "timeout"
 
 module MCP
   class StringUtilsTest < Minitest::Test
@@ -18,5 +19,18 @@ module MCP
       assert_equal("test", StringUtils.handle_from_class_name("Module::Submodule::Test"))
       assert_equal("test_class", StringUtils.handle_from_class_name("Module::Submodule::TestClass"))
     end
+
+    def test_handle_from_class_name_does_not_cause_redos
+      # A long string of uppercase letters followed by a non-lowercase character
+      # would trigger catastrophic backtracking with the vulnerable regex patterns.
+      malicious_input = "A" * 50_000 + "!"
+
+      result = nil
+      Timeout.timeout(1) do
+        result = StringUtils.handle_from_class_name(malicious_input)
+      end
+
+      assert_equal("a" * 50_000 + "!", result)
+    end
   end
 end