Stop populating the write-only namespace description in desc

[ericproulx]

desc wrote its settings into two places — the route scope (route_setting(:description)) and the namespace scope (namespace_setting(:description)) — but the namespace copy was pure dead state:

• Grape's runtime reads only route[:description] (in route(), and only for the :params key).
• The namespace scope isn't wired for inheritance — InheritableSetting#inherit_from sets inherited_values for namespace_inheritable/namespace_stackable/namespace_reverse_stackable and merges route, but never touches @namespace. So namespace-scope values are only cloned by point_in_time_copy and never consulted.
• grape-swagger doesn't read it either (checked against the local checkout).

Confirmed by experiment: dropping the write breaks only the desc_spec.rb assertions that checked the write itself — the rest of the suite is unaffected.

Change

desc now writes only the route scope. namespace_setting(:description) returns nil; a route's description remains available via route_setting(:description) and the route.description reader. UPGRADING note added.

Full suite green (2351 examples, 0 failures); RuboCop clean.

🤖 Generated with Claude Code

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[dblock]

Can a namespace have its own/different description?

[ericproulx]

No — and it couldn't before this PR either. desc has only ever described the next endpoint: the runtime consumes only route[:description], and the namespace-scope copy removed here was write-only (the @namespace scope isn't wired into inherit_from, and neither Grape nor grape-swagger ever read it).

What desc before a namespace actually does is unchanged by this PR — it attaches to the first route defined inside (verified against this branch):

desc 'namespace-level description'
namespace :users do
  get :first   # description: "namespace-level description"

  desc 'route-level description'
  get :second  # "route-level description"

  get :third   # nil
end

So routes inside a namespace can each have their own description, exactly as before. For metadata on the namespace itself, the supported channel is options on the namespace call — namespace :users, desc: 'User operations' — which land in Grape::Namespace#options and are what grape-swagger walks via namespace_stackable[:namespace].

(Side note: an unconsumed desc also leaks into the first route of the next sibling namespace — a pre-existing quirk, untouched here; happy to open a follow-up issue.)

[dblock]

An unconsumed desc also leaks into the first route of the next sibling namespace — a pre-existing quirk, untouched here; happy to open a follow-up issue.

Yes, this looks unexpected. I think desc on a namespace should cause an error.