Document open ranges as a lighter alternative to a numericality validator

[dblock]

Summary

#2822 proposed a new numericality validator for bounding numeric parameters (greater_than, less_than, equal_to, etc.). As discussed on that PR, Grape's existing :values option already supports Range, including open (beginless/endless) ranges, since it validates with Range#include?. This covers the same common use cases without adding a new validator:

params do
  requires :quantity, type: Integer, values: 1..                # must be a positive integer
  requires :discount, type: Float, values: 0.0..100.0           # must be between 0 and 100
  requires :rating,   type: Integer, values: 5..5                # must be exactly 5
  requires :numbers,  type: [Integer], values: 1..                # every element must be positive
end

This PR does not change any validator code (open ranges already work end-to-end). It:

• Adds a /beginless range test alongside the existing /endless one in values_validator_spec.rb.
• Ports the bound-checking scenarios from Add a numericality validator #2822's numericality_validator_spec.rb (greater_than, greater_than_or_equal_to/less_than_or_equal_to range, equal_to, array element-wise checking, integer-only via type:) as values: + range equivalents.
• Documents these patterns in the README's :values section.

Test plan

• bundle exec rspec spec/grape/validations/validators/values_validator_spec.rb (73 examples, 0 failures)
• bundle exec rake (full suite + RuboCop) passes locally

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