Add bug tracker URL metadata to gemspec

It would be convenient if users could visit to the bug tracker directly from the published RubyGems.
https://rubygems.org/gems/mcp

mcp.gemspec

@@ -19,6 +19,7 @@ Gem::Specification.new do |spec|
   spec.metadata["changelog_uri"] = "https://github.com/modelcontextprotocol/ruby-sdk/releases/tag/v#{spec.version}"
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage
+  spec.metadata["bug_tracker_uri"] = "#{spec.homepage}/issues"
 
   spec.files = Dir.chdir(File.expand_path("..", __FILE__)) do
     %x(git ls-files -z).split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }