Expose experiment context_keys in the experiments API

Summary

The GET /experiments API does not expose each experiment's declared context_keys. The definition object is serialized from Feature::Definition (lib/feature/shared.rb PARAMS), which has no context field, and context_keys lives on the experiment class (app/experiments/*), not the feature definition.

As a result, API consumers (notably the Experiment Dashboard) cannot tell which context keys (user, namespace, project, actor) an experiment expects when calling the force/read/clear assignment endpoints. The dashboard can therefore only operate on the current user.

Proposal

Expose a top-level context array on the Experiment entity, resolving the experiment class the same way Experiments::ContextResolver does:

expose :context do |definition|
  ApplicationExperiment.constantize(definition.attributes[:name]).context_keys
rescue Gitlab::AbstractMethodError, Gitlab::Experiment::UnregisteredExperiment
  []
end

Returns e.g. ["user"], or [] for experiments that don't declare context keys / aren't registered.

Acceptance criteria

• GET /experiments returns a context array per experiment listing its context_keys.
• Returns [] for experiments without declared context keys (no error).
• Entity + request spec coverage.
• API docs + regenerated OpenAPI.

Why

Unblocks the dashboard from rendering the correct context inputs so team members can force/clear assignments for arbitrary users/namespaces/projects (gitlab-org/growth/experiment-dashboard#5), not just themselves.

Related to #591174 (closed)