Stop populating the write-only namespace description in desc

`desc` stored its settings under both the route scope
(`route_setting(:description)`) and the namespace scope
(`namespace_setting(:description)`). The namespace copy was never read:
`route` consumes only `route[:description]`, the namespace scope is not
wired for inheritance (`InheritableSetting#inherit_from` skips `@namespace`),
and grape-swagger reads neither. `desc` now writes only the route scope.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ericproulx

CHANGELOG.md

@@ -12,6 +12,7 @@
 * [#2782](https://github.com/ruby-grape/grape/pull/2782): Remove the dead `format` keyword from `Grape::Router::Pattern` - [@ericproulx](https://github.com/ericproulx).
 * [#2783](https://github.com/ruby-grape/grape/pull/2783): Read a route's `version`, `anchor` and `requirements` from its pattern instead of storing them again on the route - [@ericproulx](https://github.com/ericproulx).
 * [#2784](https://github.com/ruby-grape/grape/pull/2784): Move HEAD route creation into `Grape::Router::Route#to_head` - [@ericproulx](https://github.com/ericproulx).
+* [#2787](https://github.com/ruby-grape/grape/pull/2787): Stop populating the write-only `namespace_setting(:description)` in `desc` - [@ericproulx](https://github.com/ericproulx).
 * Your contribution here.
 
 #### Fixes

UPGRADING.md

@@ -60,6 +60,10 @@ The `:format` member was removed from the endpoint's internal `Options`. Nothing
 
 `Grape::Router::Pattern#initialize` no longer accepts `format:` either. It was never given a non-`nil` value — a route's `:format` capture comes from the path suffix built by `Grape::Path`, not from the pattern — so the keyword was dead. `Grape::Router::Pattern.new(..., format: …)` now raises `unknown keyword: :format`.
 
+#### `desc` no longer populates `namespace_setting(:description)`
+
+`desc` used to store its settings under both the route scope (`route_setting(:description)`) and the namespace scope (`namespace_setting(:description)`). The namespace copy was write-only — the namespace scope isn't wired for inheritance and nothing in Grape or grape-swagger ever read it — so `desc` now writes only the route scope. `namespace_setting(:description)` returns `nil`; read a route's description through `route_setting(:description)` or the `route.description` reader.
+
 ### Upgrading to >= 3.3
 
 #### Minimum required Ruby is now 3.3

lib/grape/dsl/desc.rb

@@ -63,7 +63,8 @@ def desc(description, *legacy_options, **options, &config_block)
           else
             options.merge(description:)
           end
-        inheritable_setting.namespace[:description] = settings
+        # Only the route scope is consumed (by +route+, for its +:params+);
+        # the namespace scope was write-only, so it is no longer populated.
         inheritable_setting.route[:description] = settings
       end
     end

spec/grape/dsl/desc_spec.rb

@@ -15,8 +15,8 @@
       desc_text = 'The description'
       options = { message: 'none' }
       subject.desc desc_text, **options
-      expect(subject.namespace_setting(:description)).to eq(options.merge(description: desc_text))
       expect(subject.route_setting(:description)).to eq(options.merge(description: desc_text))
+      expect(subject.namespace_setting(:description)).to be_nil
     end
 
     context 'when a positional options Hash is passed' do
@@ -31,8 +31,8 @@
         Grape.deprecator.silence do
           subject.desc desc_text, options
         end
-        expect(subject.namespace_setting(:description)).to eq(options.merge(description: desc_text))
         expect(subject.route_setting(:description)).to eq(options.merge(description: desc_text))
+        expect(subject.namespace_setting(:description)).to be_nil
       end
     end
 
@@ -103,8 +103,8 @@
           security %w[array of security schemes]
         end
 
-        expect(subject.namespace_setting(:description)).to eq(expected_options)
         expect(subject.route_setting(:description)).to eq(expected_options)
+        expect(subject.namespace_setting(:description)).to be_nil
       end
     end
   end