Fix an incorrect auto-correct for Style/SingleLineMethods

Fix the following incorrect auto-correct for `Style/SingleLineMethods`
when using single line class method definition.

```console
% cat example.rb
def self.some_method; body end

% rubocop --only Style/SingleLineMethods -a
(snip)

Offenses:

example.rb:1:1: C: [Corrected] Style/SingleLineMethods: Avoid
single-line method definitions.
def self.some_method; body end
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

1 file inspected, 1 offense detected, 1 offense corrected
```

## Before

```console
% cat example.rb
def some_method() = body
```

`self` is lost due to auto-correction.

## After

```console
% cat example.rb
def self.some_method() = body
```

changelog/fix_incorrect_autocorrect_for_style_single_line_methods.md

@@ -0,0 +1 @@
+* [#9645](https://github.com/rubocop/rubocop/pull/9645): Fix an incorrect auto-correct for `Style/SingleLineMethods` when using single line class method definition. ([@koic][])

lib/rubocop/cop/style/single_line_methods.rb

@@ -91,8 +91,9 @@ def correct_to_multiline(corrector, node)
         end
 
         def correct_to_endless(corrector, node)
+          self_receiver = node.self_receiver? ? 'self.' : ''
           arguments = node.arguments.any? ? node.arguments.source : '()'
-          replacement = "def #{node.method_name}#{arguments} = #{node.body.source}"
+          replacement = "def #{self_receiver}#{node.method_name}#{arguments} = #{node.body.source}"
           corrector.replace(node, replacement)
         end
 

spec/rubocop/cop/style/single_line_methods_spec.rb

@@ -168,6 +168,12 @@ def some_method() = body
         RUBY
       end
 
+      it 'corrects to an endless class method definition' do
+        expect_correction(<<~RUBY.strip, source: 'def self.some_method; body end')
+          def self.some_method() = body
+        RUBY
+      end
+
       it 'retains comments' do
         source = 'def some_method; body end # comment'
         expect_correction(<<~RUBY.strip, source: source)