Wrap Rack parameter parsing errors in Grape::Exceptions::RequestError

[ericproulx]

Closes #2668

Summary

Replaces five specific Rack-related exception classes with a single Grape::Exceptions::RequestError that forwards the Rack exception's own message directly, removing Grape's translated messages for these cases.

Changes

New: Grape::Exceptions::RequestError

A lightweight wrapper that takes any exception and an optional status: (default 400), forwarding the exception's message as-is.

Removed exception classes

Removed	Was raised for
Grape::Exceptions::EmptyMessageBody	EOFError
Grape::Exceptions::TooManyMultipartFiles	Rack::Multipart::MultipartPartLimitError/TotalPartLimitError
Grape::Exceptions::TooDeepParameters	Rack::QueryParser::ParamsTooDeepError
Grape::Exceptions::ConflictingTypes	Rack::Utils::ParameterTypeError
Grape::Exceptions::InvalidParameters	Rack::Utils::InvalidParameterError

Grape::RACK_ERRORS constant

Centralises the list of rescuable Rack exceptions. On Rack 3, Rack::BadRequest is a marker module included by all bad-request exception classes — a single rescue entry covers them all. On Rack 2, each class is listed individually.

# Rack 3
RACK_ERRORS = [EOFError, Rack::BadRequest]

# Rack 2
RACK_ERRORS = [EOFError, Rack::Multipart::MultipartPartLimitError, ...]

Grape::RACK_ERRORS is also used in Grape::Middleware::Base#query_params for consistent handling.

Breaking changes

• The five removed exception classes no longer exist. Rescue Grape::Exceptions::RequestError instead.
• Error messages for these cases now come directly from Rack rather than Grape's locale file. The corresponding translation keys have been removed from en.yml.

See UPGRADING.md for migration instructions.

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[dblock]

Let's also add CI for the newer Rack version explicitly (vs. latest)?