Support Ruby 2.7 - Ruby 3.1

## Summary

Unlike applications, libraries provide value by remaining usable for a broad range of users,
even when that means supporting older Ruby versions beyond Ruby's own EOL timeline.

I maintain RuboCop, a Ruby linter. Because linters need to work across the ecosystem,
they often need to keep compatibility with older Ruby versions to some extent.

The trade-off is that the MCP Ruby SDK must remain compatible with Ruby 2.7.
However, given the ecosystem impact, supporting Ruby 2.7 is still worthwhile.

For example, bundled gems such as `csv` (Ruby >= 2.5), `bigdecimal` (Ruby >= 2.5), `json` (Ruby >= 2.7),
and `language_server-protocol` (Ruby >= 2.5) tend to keep compatibility with older Ruby versions.

Unlike dropping support, which imposes restrictions on users,
this change broadens compatibility and does not introduce breaking changes.

## Development Note

`rubocop-shopify` (2.18) supports Ruby 3.1+, and this PR does not change that.
Since RuboCop can configure the runtime Ruby version and the target Ruby version for analysis independently,
CI continues to run RuboCop on Ruby 4.0, while analyzing code with `TargetRubyVersion: 2.7`.

When setting `TargetRubyVersion: 2.7`, several RuboCop offenses were resolved.

Additionally, Sorbet-related libraries support Ruby 3.0+. Since these are not dependencies of the MCP Ruby SDK itself,
they are versioned and tested in the development Gemfile.

test/mcp/server_test.rb

@@ -80,7 +80,7 @@ module MCP
         prompts: [@prompt],
         resources: [@resource],
         resource_templates: [@resource_template],
-        configuration:,
+        configuration: configuration,
       )
     end
 
@@ -244,7 +244,13 @@ module MCP
       tool_args = { arg: "value" }
       tool_response = Tool::Response.new([{ result: "success" }])
 
-      @tool.expects(:call).with(arg: "value", server_context: nil).returns(tool_response)
+      if RUBY_VERSION >= "3.1"
+        # Ruby 3.1+: Mocha stub preserves `method.parameters` info.
+        @tool.expects(:call).with(arg: "value", server_context: nil).returns(tool_response)
+      else
+        # Ruby 3.0: Mocha stub changes `method.parameters`, so `accepts_server_context?` returns false.
+        @tool.expects(:call).with(arg: "value").returns(tool_response)
+      end
 
       request = {
         jsonrpc: "2.0",
@@ -258,7 +264,7 @@ module MCP
 
       response = @server.handle(request)
       assert_equal tool_response.to_h, response[:result]
-      assert_instrumentation_data({ method: "tools/call", tool_name: })
+      assert_instrumentation_data({ method: "tools/call", tool_name: tool_name })
     end
 
     test "#handle tools/call returns error response with isError true if required tool arguments are missing" do
@@ -296,7 +302,13 @@ module MCP
       tool_args = { arg: "value" }
       tool_response = Tool::Response.new([{ result: "success" }])
 
-      @tool.expects(:call).with(arg: "value", server_context: nil).returns(tool_response)
+      if RUBY_VERSION >= "3.1"
+        # Ruby 3.1+: Mocha stub preserves `method.parameters` info.
+        @tool.expects(:call).with(arg: "value", server_context: nil).returns(tool_response)
+      else
+        # Ruby 3.0: Mocha stub changes `method.parameters`, so `accepts_server_context?` returns false.
+        @tool.expects(:call).with(arg: "value").returns(tool_response)
+      end
 
       request = JSON.generate({
         jsonrpc: "2.0",
@@ -308,10 +320,12 @@ module MCP
       raw_response = @server.handle_json(request)
       response = JSON.parse(raw_response, symbolize_names: true) if raw_response
       assert_equal tool_response.to_h, response[:result] if response
-      assert_instrumentation_data({ method: "tools/call", tool_name: })
+      assert_instrumentation_data({ method: "tools/call", tool_name: tool_name })
     end
 
     test "#handle_json tools/call executes tool and returns result, when the tool is typed with Sorbet" do
+      skip "Sorbet is not available" unless defined?(T::Sig)
+
       class TypedTestTool < Tool
         tool_name "test_tool"
         description "a test tool for testing"
@@ -836,7 +850,7 @@ module MCP
       configuration.instrumentation_callback = local_callback
       configuration.exception_reporter = local_exception_reporter
 
-      server = Server.new(name: "test_server", configuration:)
+      server = Server.new(name: "test_server", configuration: configuration)
 
       assert_equal local_callback, server.configuration.instrumentation_callback
       assert_equal local_exception_reporter, server.configuration.exception_reporter