Fix false positives for Style/EndlessMethod

Follow-up to #13874 (comment)

This PR fixes false positives for `Style/EndlessMethod` when using assignment method definitions.

Setter methods cannot be defined as endless methods.

```console
$ ruby -vc -e "def foo=(arg) = bar"
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +PRISM [x86_64-darwin23]
ruby: -e:1: syntax error found (SyntaxError)
> 1 | def foo=(arg) = bar
    |     ^~~~ invalid method name; a setter method cannot be defined in an endless method definition
```

changelog/fix_false_positives_for_style_endless_method.md

@@ -0,0 +1 @@
+* [#13910](https://github.com/rubocop/rubocop/pull/13910): Fix false positives for `Style/EndlessMethod` when using setter method definitions. ([@koic][])

lib/rubocop/cop/style/endless_method.rb

@@ -144,6 +144,8 @@ class EndlessMethod < Base
         MSG_REQUIRE_ALWAYS = 'Use endless method definitions.'
 
         def on_def(node)
+          return if node.assignment_method?
+
           case style
           when :allow_single_line, :allow_always
             handle_allow_style(node)

spec/rubocop/cop/style/endless_method_spec.rb

@@ -244,6 +244,14 @@ def my_method = x
         RUBY
       end
 
+      it 'does not register an offense for a single line setter method' do
+        expect_no_offenses(<<~RUBY)
+          def my_method=(arg)
+            arg.foo
+          end
+        RUBY
+      end
+
       it 'does not register an offense when the endless version excess Metrics/MaxLineLength[Max]' do
         expect_no_offenses(<<~RUBY)
           def my_method
@@ -392,6 +400,16 @@ def my_method = x.foo
         RUBY
       end
 
+      it 'does not register an offense for a multiline setter method' do
+        expect_no_offenses(<<~RUBY)
+          def my_method=(arg)
+            x.foo
+             .bar
+             .baz
+          end
+        RUBY
+      end
+
       it 'does not register an offense when the endless version excess Metrics/MaxLineLength[Max]' do
         expect_no_offenses(<<~RUBY)
           def my_method