Return 406 Not Acceptable for malformed Accept headers

[jrhoads]

Purpose

Handle malformed or invalid Accept headers so the API returns a clean 406 Not Acceptable response instead of raising an unhandled ActionDispatch::Http::MimeNegotiation::InvalidType error.

Approach

Add a custom Rack middleware that intercepts ActionDispatch::Http::MimeNegotiation::InvalidType before it propagates through the Rails instrumentation stack, and return a short plain-text error message.

Key Modifications

• Added config/initializers/catch_invalid_mime_type.rb defining CatchInvalidMimeTypeMiddleware.
• Inserted the middleware into the Rails stack after ActionDispatch::DebugExceptions.
• Added a request spec in spec/api/api_spec.rb for a malformed Accept header (application/json, *).
• Added the corresponding VCR cassette fixture for the test.

Important Technical Details

• ActionDispatch::Http::MimeNegotiation::InvalidType is raised inside ActionController::Instrumentation#process_action, which occurs before rescue_from callbacks run, so a middleware layer is the appropriate place to handle it.
• The middleware returns [406, { "Content-Type" => "text/plain" }, [RESPONSE_BODY]].
• Catching the exception inside DebugExceptions prevents noisy multi-line backtraces from being logged.
• The middleware is inserted after ActionDispatch::DebugExceptions so the exception can be handled cleanly before reaching the debug error page.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

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