Action View: Fallback to existing partial when possible

Closes [rails#50844][]

Motivation / Background
---

A controller declared in the top-level module can render a top-level
Active Model instance whose partial is declared in the root view
directory (like `articles/_article.html.erb`).

A controller scoped within a module can render an Active Model instance
whose partial is similarly scoped within view directory (like
`scoped/articles/_article.html.erb`).

A controller scoped within a module cannot render an Active Model
instance whose partial is declared in the root view directory (like
`articles/_article.html.erb`), despite the absence of a similarly scoped
partial.

This is intended behavior that's powered by
[`config.action_view.prefix_partial_path_with_controller_namespace =
true`][prefix_partial_path_with_controller_namespace] (`true` by
default).

This change was introduced in March of 2012 as part of [rails#5625][].

Detail
---

As a consumer of Action View, my intuition is that the lookup would
fallback, in the same way that a controller that inherits from
`ApplicationController` could define its own view, then rely on fallback
to render an `app/views/application` partial.

This commit modifies the behavior to gracefully fall back to the
root-level view partial.

Checklist
---

Before submitting the PR make sure the following are checked:

* [x] This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
* [x] Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: `[Fix #issue-number]`
* [x] Tests are added or updated if you fix a bug or add a feature.
* [x] CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[#59844]: rails#50844
[prefix_partial_path_with_controller_namespace]: https://guides.rubyonrails.org/configuring.html#config-action-view-prefix-partial-path-with-controller-namespace
[rails#5625]: rails#5625

actionview/CHANGELOG.md

@@ -1,3 +1,34 @@
+*   Fallback to rendering partial without controller namespace prefix when available
+
+    When `config.action_view.prefix_partial_path_with_controller_namespace =
+    true` and a root-level partial exists, fallback to rendering the partial:
+
+    ```erb
+    <%= # app/views/articles/_article.html.erb %>
+    Rendered
+    <% end %>
+    ```
+
+    ```ruby
+    # app/controllers/articles_controller.rb
+    class ArticlesController < ApplicationController
+      def show
+        render partial: Article.find(params[:id])
+        # => "Rendered"
+      end
+    end
+
+    # app/controllers/scoped/articles_controller.rb
+    class Scoped::ArticlesController < ApplicationController
+      def show
+        render partial: Article.find(params[:id])
+        # => "Rendered"
+      end
+    end
+    ```
+
+    *Sean Doyle*
+
 *   Raise `ArgumentError` if `:renderable` object does not respond to `#render_in`
 
     *Sean Doyle*

actionview/lib/action_view/renderer/object_renderer.rb

@@ -13,7 +13,18 @@ def initialize(lookup_context, options)
     def render_object_with_partial(object, partial, context, block)
       @object     = object
       @local_name = local_variable(partial)
-      render(partial, context, block)
+
+      begin
+        render(partial, context, block)
+      rescue MissingTemplate
+        _, *parts = partial.split("/")
+
+        if parts.many?
+          render_object_with_partial(@object, parts.join("/"), context, block)
+        else
+          raise
+        end
+      end
     end
 
     def render_object_derive_partial(object, context, block)

actionview/test/actionpack/controller/render_test.rb

@@ -1072,6 +1072,27 @@ def test_render_to_string_inline
     assert_equal "Hello world!", @response.body
   end
 
+  def test_unnested_rendering_with_fallback
+    @controller = Fun::GamesController.new
+    def @controller.hello_world
+      render partial: ::Customer.new("Rendered"), locals: { greeting: "Hello" }
+    end
+
+    get :hello_world
+    assert_equal "Hello: Rendered", @response.body
+  end
+
+  def test_unnested_rendering_without_fallback
+    @controller = Fun::GamesController.new
+    def @controller.hello_world
+      render partial: Post.new
+    end
+
+    assert_raises ActionView::MissingTemplate, match: "Missing partial posts/_post" do
+      get :hello_world
+    end
+  end
+
   # :ported:
   def test_nested_rendering
     @controller = Fun::GamesController.new