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Resolve the params builder when params are first read, not in the constructor (#2892)

`Grape::Request#initialize` runs once per request, and resolved the params
builder eagerly:

    def initialize(env, build_params_with: nil)
      super(env)
      @params_builder = Grape::ParamsBuilder.params_builder_for(build_params_with || Grape.config[:param_builder])
    end

Two lookups behind that line, both on the request path:

* `Grape.config[:param_builder]` — dry-configurable's `Config#[]`, 126 ns;
* `params_builder_for` — a `key?` plus a `[]` against the registry
  `HashWithIndifferentAccess`, 200 ns together.

`@params_builder` has exactly one reader, `make_params`, which only runs when
something asks for `params`. An endpoint with no validations whose block never
mentions them — a health check, a static resource, a redirect — paid both
lookups to build an object it then dropped.

Memoize it on first use instead. `Grape.config` stays the source of truth, read
at the same point in the request as before for anything that does read params,
so a `build_with :unknown` still surfaces as a 500 during the request rather
than at mount time.

Median of 5 interleaved runs against master (Ruby 4.0.5): **+5.9%** on a
minimal versioned GET that never touches params; **+1.1%** on one with params
and validations, where the work moved rather than disappeared.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

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