Add a numericality validator

[ericproulx]

Summary

Grape currently has no built-in validator for bounding or shape-checking a numeric parameter beyond values: (a closed inclusive range or an explicit list/predicate). Common cases like "must be a positive integer", "0 to 100 with an exclusive upper bound", or "must be a perfect score of exactly 5" require either abusing values: with a lambda (weak error messages, awkward for open-ended bounds) or writing a full contract_scope schema for a single field.

This PR adds a numericality validator, following ActiveModel::Validations::NumericalityValidator's option names for familiarity to anyone coming from Rails:

params do
  requires :quantity, type: Integer, numericality: { greater_than: 0 }
  requires :discount, type: Float, numericality: { greater_than_or_equal_to: 0, less_than_or_equal_to: 100 }
  requires :rating,   type: Integer, numericality: { equal_to: 5 }
  requires :page,     type: Integer, numericality: { greater_than: 0, only_integer: true }
end

Supported options (any combination): greater_than, greater_than_or_equal_to, less_than, less_than_or_equal_to, equal_to, other_than, only_integer, odd, even. Each has its own i18n message key (numericality_greater_than, etc.), mirroring how length already has separate length_min/length_max/length_is keys. Per-field message: overrides work the same way as every other validator.

Design notes

• Non-Numeric values are skipped, not errored. Numeric-ness is type:/coercion's job; this validator only owns bounds/parity, so it composes with coerce rather than duplicating its work. A param with no type:/coerce still round-trips through unchanged if it isn't already a Numeric.
• Typed arrays are checked element-wise. requires :numbers, type: [Integer], numericality: { greater_than: 0 } validates every element (via Array.wrap), matching how values already treats arrays. length is the one validator that intentionally owns the array's own size, so no overlap there.
• only_integer accepts whole-numbered Float/BigDecimal, not just literal Integer — val == val.to_i — so type: Float, numericality: { only_integer: true } accepts 5.0 and rejects 5.5.
• Obviously contradictory combinations raise ArgumentError at definition time, not at request time: greater_than + greater_than_or_equal_to, less_than + less_than_or_equal_to, odd + even, equal_to + any other comparison option, and a reversed lower/upper bound (e.g. greater_than: 10, less_than: 5). This mirrors LengthValidator's existing min > max guard.

No existing validator's behavior changes; this is purely additive (new validator class, new locale keys, new README section). No UPGRADING.md entry needed.

Test plan

• New spec/grape/validations/validators/numericality_validator_spec.rb (34 examples): every option's pass/fail case, the range combination, the only_integer Float case, odd/even, array-element-wise checking, non-Numeric skip behavior, allow_blank interplay, custom message: override, and every ArgumentError guard.
• Full RSpec suite passes locally (2401 examples, 0 failures).
• RuboCop clean across the repo.
• CI green.

🤖 Generated with Claude Code

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[dblock]

Supporting open ranges would be cleaner because we already support closed ranges, no?

params do
  requires :quantity, type: Integer, values: 1.. # numericality: { greater_than: 0 }
  requires :discount, type: Float, values: 0..100 # numericality: { greater_than_or_equal_to: 0, less_than_or_equal_to: 100 }
  requires :rating,   type: Integer, values: 5 # numericality: { equal_to: 5 }
  requires :page,     type: Integer, values: 1.. # this one has to be integer because of type ... numericality: { greater_than: 0, only_integer: true }
end

[ericproulx]

Yeah, makes sense. I'll drop it.

[dblock]

Yeah, makes sense. I'll drop it.

Will you add support for open ranges? It's a useful feature.

Copilot is working ...

[dblock]

#2831