[Fix rubocop#10685] Fix a false positive for Style/StringConcatenation

Fixes rubocop#10685.

This PR fixes a false positive for `Style/StringConcatenation`
when `Mode: conservative` and first operand is not string literal.

This also resolves the following feedback:
rubocop#9153 (comment)

`Mode: conservative` should be allowed considering the possibility that
receiver is a `Pathname` object.

changelog/fix_a_false_positive_for_style_string_concatenation.md

@@ -0,0 +1 @@
+* [#10685](https://github.com/rubocop/rubocop/issues/10685): Fix a false positive for `Style/StringConcatenation` when `Mode: conservative` and first operand is not string literal. ([@koic][])

lib/rubocop/cop/style/string_concatenation.rb

@@ -76,7 +76,7 @@ def on_send(node)
 
           topmost_plus_node = find_topmost_plus_node(node)
           parts = collect_parts(topmost_plus_node)
-          return unless parts[0..-2].any? { |receiver_node| offensive_for_mode?(receiver_node) }
+          return if mode == :conservative && !parts.first.str_type?
 
           register_offense(topmost_plus_node, parts)
         end
@@ -95,11 +95,6 @@ def register_offense(topmost_plus_node, parts)
           end
         end
 
-        def offensive_for_mode?(receiver_node)
-          mode = cop_config['Mode'].to_sym
-          mode == :aggressive || (mode == :conservative && receiver_node.str_type?)
-        end
-
         def line_end_concatenation?(node)
           # If the concatenation happens at the end of the line,
           # and both the receiver and argument are strings, allow
@@ -173,6 +168,10 @@ def handle_quotes(parts)
         def single_quoted?(str_node)
           str_node.source.start_with?("'")
         end
+
+        def mode
+          cop_config['Mode'].to_sym
+        end
       end
     end
   end

spec/rubocop/cop/style/string_concatenation_spec.rb

@@ -224,6 +224,7 @@
         expect_no_offenses(<<~RUBY)
           user.name + "!!"
           user.name + "<"
+          user.name + "<" + "user.email" + ">"
         RUBY
       end
     end
@@ -235,14 +236,11 @@
           ^^^^^^^^^^^^^^^^^^^^ Prefer string interpolation to string concatenation.
           "Hello " + user.name + "!!"
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer string interpolation to string concatenation.
-          user.name + "<" + "user.email" + ">"
-          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer string interpolation to string concatenation.
         RUBY
 
         expect_correction(<<~RUBY)
           "Hello \#{user.name}"
           "Hello \#{user.name}!!"
-          "\#{user.name}<user.email>"
         RUBY
       end
     end