Fix build error

Follow-up to #27.

This PR fixes the following build error:

```console
$ bundle exec rake
/Users/koic/.rbenv/versions/3.5-dev/lib/ruby/3.5.0+1/bundled_gems.rb:59:in 'Kernel.require': cannot load such file -- mcp/transports/stdio (LoadError)
Did you mean?  mcp/transports/stdio_transport_test
        from /Users/koic/.rbenv/versions/3.5-dev/lib/ruby/3.5.0+1/bundled_gems.rb:59:in 'block (2 levels) in Kernel#replace_require'
        from /Users/koic/src/github.com/modelcontextprotocol/ruby-sdk/test/mcp/transports/stdio_transport_test.rb:4:in '<top (required)>'
        from /Users/koic/.rbenv/versions/3.5-dev/lib/ruby/3.5.0+1/bundled_gems.rb:59:in 'Kernel.require'
        from /Users/koic/.rbenv/versions/3.5-dev/lib/ruby/3.5.0+1/bundled_gems.rb:59:in 'block (2 levels) in Kernel#replace_require'
        from /Users/koic/.rbenv/versions/3.5-dev/lib/ruby/gems/3.5.0+1/gems/rake-13.3.0/lib/rake/rake_test_loader.rb:21:in 'block in <main>'
        from /Users/koic/.rbenv/versions/3.5-dev/lib/ruby/gems/3.5.0+1/gems/rake-13.3.0/lib/rake/rake_test_loader.rb:6:in 'Array#select'
        from /Users/koic/.rbenv/versions/3.5-dev/lib/ruby/gems/3.5.0+1/gems/rake-13.3.0/lib/rake/rake_test_loader.rb:6:in '<main>'
rake aborted!
Command failed with status (1)
/Users/koic/.rbenv/versions/3.5-dev/bin/bundle:25:in 'Kernel#load'
/Users/koic/.rbenv/versions/3.5-dev/bin/bundle:25:in '<main>'
Tasks: TOP => default => test
(See full trace by running task with --trace)
```

https://github.com/modelcontextprotocol/ruby-sdk/actions/runs/15428236624/job/43420346011

The following is stated in #27.

> Breaking Changes
> `MCP::Transports::StdioTransport` is now `MCP::Server::Transports::StdioTransport`

What is shown here is the new name `MCP::Server::Transports::StdioTransport`, not `MCP::Server::Transports::Stdio`.

Therefore, the file path has been updated from `lib/mcp/server/transports/stdio.rb` to `lib/mcp/server/transports/stdio_transport.rb` to
match the module name. If `MCP::Server::Transports::Stdio` is preferred instead, please let me know and I will open a separate PR.

Author: koic

README.md

@@ -86,7 +86,7 @@ If you want to build a local command-line application, you can use the stdio tra
 ```ruby
 #!/usr/bin/env ruby
 require "mcp"
-require "mcp/server/transports/stdio"
+require "mcp/server/transports/stdio_transport"
 
 # Create a simple tool
 class ExampleTool < MCP::Tool

examples/stdio_server.rb

@@ -3,7 +3,7 @@
 
 $LOAD_PATH.unshift(File.expand_path("../lib", __dir__))
 require "mcp"
-require "mcp/server/transports/stdio"
+require "mcp/server/transports/stdio_transport"
 
 # Create a simple tool
 class ExampleTool < MCP::Tool

lib/mcp.rb

@@ -13,7 +13,7 @@
 require_relative "mcp/resource/embedded"
 require_relative "mcp/resource_template"
 require_relative "mcp/server"
-require_relative "mcp/server/transports/stdio"
+require_relative "mcp/server/transports/stdio_transport"
 require_relative "mcp/string_utils"
 require_relative "mcp/tool"
 require_relative "mcp/tool/input_schema"

lib/mcp/server/transports/stdio.rb renamed to lib/mcp/server/transports/stdio_transport.rb

@@ -1,13 +1,13 @@
 # frozen_string_literal: true
 
 require "test_helper"
-require "mcp/server/transports/stdio"
+require "mcp/server/transports/stdio_transport"
 require "json"
 
 module MCP
   class Server
     module Transports
-      class StdioTest < ActiveSupport::TestCase
+      class StdioTransportTest < ActiveSupport::TestCase
         include InstrumentationTestHelper
 
         setup do