Make ErrorFormatter.formatter_for a lookup and move the fallback to its caller

[ericproulx]

Stacked on #2901 — review that one first; this PR's base is error-formatter-nil-guard and I'll retarget it to master once #2901 merges.

ErrorFormatter.formatter_for took the API's default_error_formatter as a third argument and ended in default_error_formatter || Grape::ErrorFormatter::Txt, so the global registry implemented the caller's precedence out of the caller's own state. It was the only registry in the family that did:

module	signature	miss returns
Parser.parser_for	(format, parsers = nil)	nil — caller decides
ParamsBuilder.params_builder_for	(short_name)	raises
Formatter.formatter_for	(api_format, formatters)	DEFAULT_LAMBDA_FORMATTER (its own constant)
ErrorFormatter.formatter_for	(format, error_formatters, default_error_formatter)	default_error_formatter || Txt

error_formatters is at least a table it can search; default_error_formatter is API policy threaded through a global module. The || Txt alone would be fine — Formatter does the same — but it cannot stay once the other leaves: a caller can only insert its own default between "nothing registered" and Txt if the lookup reports the miss.

So formatter_for takes the Parser.parser_for shape, and Middleware::Error applies the fallback it owns:

# error_formatter.rb
def formatter_for(format, error_formatters = nil)
  return error_formatters[format] if error_formatters&.key?(format)

  registry[format]
end

# middleware/error.rb — Options#initialize, next to the existing rescue_options ||=
default_error_formatter ||= Grape::ErrorFormatter::Txt

# format_message
formatter = Grape::ErrorFormatter.formatter_for(current_format, error_formatters) || default_error_formatter

Options#initialize is the right home for the default because Endpoint#error_middleware_options passes an explicit nil when the API called no format, which bypasses the Data keyword default — the same reason rescue_options ||= is already there.

Resolution is unchanged, precedence included: a formatter registered for the requested format still wins over the API's default_error_formatter, which applies to formats with none of their own (format == nil included — that is what makes the default_error_formatter :json spec pass). Both directions are now pinned by specs in error_spec.rb, and a new spec/grape/error_formatter_spec.rb pins the lookup contract, mirroring parser_spec.rb.

Reporting the miss also lets the DSL see it

default_error_formatter names an error formatter, so a name nothing is registered under is a mistake. It used to store the Txt the lookup substituted, so the typo read back as a working setting and errors quietly rendered as text:

default_error_formatter :jsonn
# => Grape::Exceptions::UnknownErrorFormatter: unknown error formatter: jsonn

format names a format, where having no error formatter of its own is ordinary (format :xls), so it still succeeds and stores nil, with the middleware supplying Txt at render time.

Both are in UPGRADING, along with the signature change: a third argument to formatter_for now raises ArgumentError, and the setting reads back nil instead of Txt for a format with no error formatter — visible only if you read it back, since error responses are unchanged.

🤖 Generated with Claude Code

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