Support launching with rackup on Rack 3

[pratyush07-hub]

Description

Currently, Rage works correctly on Rack 2 but fails on Rack 3 due to the move of the handler registry to the rackup gem and the new Rackup::Handler namespace.
This PR adds support for launching Rage applications using the rackup command on Rack 3.

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Changes

• Added Rage::Handler: A new handler that delegates to the optimized Rage::CLI server logic, ensuring Rage-specific features like Fiber-concurrency are active.
• Handler Registration: Registered the new handler in both Rack::Handler (Rack 2) and Rackup::Handler (Rack 3) within lib/rage-rb.rb.
• Discovery Support: Added discovery files in lib/rack/handler/rage.rb and lib/rackup/handler/rage.rb to enable automatic identification of the Rage handler via the -s rage flag.

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Verification

• Verified the registration logic via a dedicated verification script.
• Confirmed that the rackup command correctly identifies the Rage handler when the rackup gem is present.

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Closes #259

[pratyush07-hub]

Hi @rsamoilov , This PR is now ready for your review. PTAL!

[rsamoilov]

Hi @pratyush07-hub

I can see the following error when trying to run rackup with your PR:

/Users/r/.gem/ruby/3.4.1/gems/rackup-2.3.1/lib/rackup/server.rb:269:in 'block in Rackup::Server.logging_middleware': undefined method 'name' for nil (NoMethodError)

          /CGI/.match?(server.server.name) || server.options[:quiet] ? nil : [Rack::CommonLogger, $stderr]

Am I doing something wrong?

Also, there's an existing Rack handler in Iodine - https://github.com/rage-rb/iodine/blob/master/lib/rack/handler/iodine.rb. Do you think these changes should go in there instead?

[pratyush07-hub]

@rsamoilov , The NoMethodError occurred because the handler wasn't explicitly registered in the discovery files, causing it to be nil in some loading scenarios. I have fixed this by adding explicit registration to lib/rack/handler/rage.rb and lib/rackup/handler/rage.rb, and added a name method to Rage::Handler.

Regarding the move to iodine, keeping the rage handler allows for better branding (rackup-s rage) and gives us direct control over Rage-specific startup logic. Furthermore, providing discovery files in rage makes the handler immediately available without requiring users to know about the internal relationship with iodine.

Additionally, rage-iodine currently lacks Rackup (Rack 3) registration, so having it here ensures full support for the latest Rack versions.
I have updated the PR with these changes.