Add Rails example app

## Motivation and Context

The README.md documents the "Rails (mount)" integration pattern, but the repository
had no runnable Rails example. This adds a minimal Rails application under examples/rails
that mounts StreamableHTTPTransport at /mcp and demonstrates class-based tools and
a resource with a read handler.

The MCP server is built in config/routes.rb rather than in an initializer because
Zeitwerk sets up autoloading after initializers run, so the tool classes in app/tools
cannot be referenced there.
Code reloading is disabled because the transport holds the server built at boot.
The app is booted with puma instead of rackup because rackup inserts `Rack::Lint`
in its development default, which rejects the capitalized response header names
that the transport currently emits.

Closes #44.

## How Has This Been Tested?

The curl walkthrough in examples/rails/README.md has been verified end to end against
the running app: initialize and the initialized notification, tools/list, tools/call for
both tools, resources/list, resources/read, the standalone SSE stream,
and session termination via DELETE.

## Breaking Changes

None. This only adds an example application and documentation links.

README.md

@@ -155,6 +155,8 @@ end
 the [MCP Streamable HTTP transport spec](https://modelcontextprotocol.io/specification/latest/basic/transports#streamable-http),
 so no additional route configuration is needed.
 
+A complete runnable application using this approach is available in [`examples/rails`](examples/rails).
+
 ##### Rails (controller)
 
 While the mount approach creates a single server at boot time, the controller approach creates a new server per request.