Update custom renderers to overload render

[rsamoilov]

This pull request introduces a redesign of the custom renderer system. Instead of generating separate render_<name> methods for each custom renderer, the system now overloads the main render method in controllers to support custom renderers via keyword arguments.

ref #244

[rsamoilov]

Hey @anuj-pal27

I've been thinking about the render_* approach more, and after an extensive grilling session with Gemini I decided to update your implementation to overload the render method instead. The main argument is discoverability - seeing render something: true leads the developer to check the documentation for render and discover a link to the custom renderers functionality. Seeing render_something doesn't tell a developer not familiar with the framework anything.

Would love your review!

[anuj-pal27]

Hey @rsamoilov — I reviewed the new approach of routing custom renderers through render (e.g. render csv: ...) and I understand the discoverability motivation, plus it keeps the status/body/double-render logic in one place.

One thing I want to double-check: if a renderer block sets headers["content-type"] (e.g. "text/html") and the implementation then delegates to render plain: result.to_s, does the plain branch still overwrite the content-type to "text/plain; charset=utf-8"?

[rsamoilov]

No, it doesn't - the changes you've implemented are still there.