Support launching with `rackup` on Rack 3

[rsamoilov]

Description

Currently, launching a Rage application with rackup works correctly on Rack 2 but fails on Rack 3. Since Rack 3 is the current stable version, this should be supported.

Tips

• Check the Rack 3 upgrade guide for breaking changes between Rack 2 and Rack 3.
• Compare how the rackup command initializes the server in Rack 2 vs Rack 3 — the handler/server registration interface has changed.
• Check the architecture doc that shows how Rage's core components interact with each other and outlines the design principles.
• Feel free to ask any questions or request help in the comments below!

[pratyush07-hub]

Hey @rsamoilov , I would like to work on this issue.

From what I understand, Rack 3 has moved rackup into a separate gem and updated handler registration to use the Rackup::Handler namespace. I’m planning to add support for this in Rage by implementing the required handler registration and adjusting the file structure for compatibility with Rack 3.

Could you please assign this issue to me?