Resolve a scope's request params in ParamsScope instead of the params DSL

[ericproulx]

Grape::DSL::Parameters holds the methods a params block calls (requires, optional, given, use, …). It also held three things that run on every request to resolve a scope's params:

• #params, called by every validator, by AttributesIterator and by ParamsScope#meets_dependency?;
• the private #map_params, which ParamsScope#validated_params called across the module boundary;
• EmptyOptionalValue, the placeholder for an Array element that left an optional scope out. The validators and AttributesIterator named it as Grape::DSL::Parameters::EmptyOptionalValue.

All three read ParamsScope's state (@parent, @element, @optional, @independent), and ParamsScope is the only class that includes the module.

Change

• #params, #map_params and the placeholder move to ParamsScope, unchanged. The placeholder is now Grape::Validations::ParamsScope::EmptyOptionalValue, documented where it is defined.
• AttributesIterator#skip? tests for it with equal?, as the validators do. It used to ask the element's ==, which for a placeholder only identity can match.
• DSL::Parameters is left with the params DSL alone.

Grape::DSL::Parameters::EmptyOptionalValue was a public constant (since 1.5.1) but never documented. GitHub code search finds it only in Grape and its forks, so it is moved rather than aliased, as #3009 did with the attribute iterator subclasses. No UPGRADING entry.

Tests

• The #params examples in dsl/parameters_spec.rb ran against a stub of the module with element and parent assigned by hand. They now run against real scopes in params_scope_spec.rb, each nested scope captured from inside its own block. The stub loses its now-unused element/parent accessors.
• Same example count as master. The ported examples pass on master's lib too, as a move should.
• Mutation-checked map_params: each of the Hash, Array and fallback branches, and the root case, fails at least one ported example when broken.

Performance

benchmark-ips on API.call(env) for a POST with a 10-element Array block holding an optional nested Hash scope, Ruby 4.0.7, master and this branch run alternately, mean of two rounds:

	master	branch
interpreter	12.92k	12.79k (−1.0%)
YJIT	37.08k	37.20k (+0.3%)

Each run's error was ±2.3–4.2%, and the interpreter result flipped between rounds, so this is noise.

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