Suppress Interrupt exception for stdio server

[koic]

Motivation and Context

This PR suppresses the following backtrace of Interrupt (Ctrl+c) exception for stdio server:

Before

$ ./examples/stdio_server.rb
^C/Users/koic/src/github.com/modelcontextprotocol/ruby-sdk/lib/mcp/server/transports/stdio_transport.rb:20:
in 'IO#gets': Interrupt
        from /Users/koic/src/github.com/modelcontextprotocol/ruby-sdk/lib/mcp/server/transports/stdio_transport.rb:20:
             in 'MCP::Server::Transports::StdioTransport#open'
        from ./examples/stdio_server.rb:95:in '<main>'
$ echo $?
130

After

$ ./examples/stdio_server.rb
^C
Exiting...
$ echo $?
130

This change should prevent unexpected backtrace from appearing when running the example: https://github.com/modelcontextprotocol/ruby-sdk?tab=readme-ov-file#stdio-transport

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed