Skip absent optional steps when composing structured error messages

[ericproulx]

Fixes #2748.

Problem

Grape::Exceptions::Base#compose_message loops over every entry in MESSAGE_STEPS (problem, summary, resolution) and translates each one individually. For an optional step a message doesn't define — e.g. summary on invalid_message_body — Grape::Util::Translation#translate falls back to returning the scoped key path as a string. That string is non-blank, so it passes the detail.present? guard and leaks verbatim into the rendered message:

Grape::Exceptions::InvalidMessageBody.new('application/json').message
# Problem:
#   message body does not match declared format
# Summary:
#   grape.errors.messages.invalid_message_body.summary   <- leaked i18n key
# Resolution:
#   when specifying application/json as content-type, ...

Fix

Gate each step on whether the already-resolved message hash defines it (short_message.key?(step.to_sym)). The resolved hash already reflects the :en fallback, so:

• absent optional steps (e.g. summary) are skipped instead of leaking the key path;
• :en locale fallback for present steps is preserved (verified under a :de locale with no override — still renders Problem + Resolution from :en, no leak);
• the intentional key-path behaviour for a fully unresolvable top-level message (spec/grape/exceptions/base_spec.rb "returns the scoped translation key as a string") is untouched — that path returns before the Hash branch;
• translate itself is unchanged, so validators and ValidationErrors are unaffected.

Tests

Added specs in spec/grape/exceptions/base_spec.rb: the leak is gone for invalid_message_body, while missing_vendor_option (which legitimately defines summary) still renders all three steps. spec/grape/exceptions/ is green (96 examples, 0 failures).

Reported by @dcasillano.

🤖 Generated with Claude Code

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