fix: handle ActionController::UnknownFormat with 406 response

[mroderick]

Summary

Adds idiomatic Rails exception handling for ActionController::UnknownFormat, returning 406 Not Acceptable for unsupported request formats.

Problem

Requests with non-HTML Accept headers (e.g., Accept: application/json from crawlers/API clients) cause ActionController::UnknownFormat exceptions when controllers don't have templates for those formats.

Example error from production:

ActionController::UnknownFormat: ChapterController#show is missing a template for this request format and variant.
request.formats: [\"application/json\"]

Solution

Uses Rails' built-in rescue_from mechanism to handle ActionController::UnknownFormat exceptions centrally in ApplicationController.

This approach was inspired by @olleolleolle's suggestion to use respond_to blocks in controllers:

respond_to do |format|
  format.html
  format.any { redirect_to support_path }
end

We adapted this to use rescue_from for a more centralized solution that follows the existing pattern in the codebase.

This is the idiomatic Rails approach, as documented in:

• Rails API: ActionController::MimeResponds
• Rails Guides: Action Controller Advanced Topics - Exception Handling

"Rails 4.0 raises ActionController::UnknownFormat when an action does not handle the requested format. By default, this exception is handled by responding with 406 Not Acceptable, but developers can now override this behavior."
— Rails Upgrade Guide

Changes

• Add rescue_from ActionController::UnknownFormat, with: :render_not_acceptable to ApplicationController
• Add render_not_acceptable method returning 406 status
• Add not_acceptable.html.haml error template
• Add comprehensive controller tests for HTML, JSON, and XML formats

Response Behavior

Format	Response
HTML	Renders errors/not_acceptable template with 406 status
JSON/XML/etc	Returns empty response with 406 status

Testing

bundle exec rspec spec/controllers/application_controller_spec.rb

All 66 controller tests pass.

Related

• Replaces: fix: ignore Accept header to prevent UnknownFormat errors #2585 (which used global config approach)
• Thanks to @olleolleolle for the respond_to suggestion that led to this solution!