Suppress Interrupt exception for stdio server

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

## Before

```console
$ ./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

```console
$ ./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

Author: koic

lib/mcp/server/transports/stdio_transport.rb

@@ -7,6 +7,8 @@ module MCP
   class Server
     module Transports
       class StdioTransport < Transport
+        STATUS_INTERRUPTED = Signal.list["INT"] + 128
+
         def initialize(server)
           @server = server
           @open = false
@@ -20,6 +22,10 @@ def open
           while @open && (line = $stdin.gets)
             handle_json_request(line.strip)
           end
+        rescue Interrupt
+          warn("\nExiting...")
+
+          exit(STATUS_INTERRUPTED)
         end
 
         def close