[ruby/syntax_suggest] Fix missing line break due to puts logic

In #225 it was reported that the output looks incorrect:

```
$ cat /tmp/4a71c7e417cc9eac0971e3a2519b295c/scratch.rb
def x.y.z
end
$ ruby /tmp/4a71c7e417cc9eac0971e3a2519b295c/scratch.rb
/tmp/4a71c7e417cc9eac0971e3a2519b295c/scratch.rb: --> /tmp/4a71c7e417cc9eac0971e3a2519b295c/scratch.rb
expected a delimiter to close the parametersunexpected '.', ignoring it
> 1  def x.y.z
> 2  end
```

Specifically:

```
expected a delimiter to close the parametersunexpected '.', ignoring it
```

However this does not show up when executing the debug executable:

```
$ bin/bundle exec exe/syntax_suggest /tmp/4a71c7e417cc9eac0971e3a2519b295c/scratch.rb
--> /tmp/4a71c7e417cc9eac0971e3a2519b295c/scratch.rb

expected a delimiter to close the parameters
unexpected '.', ignoring it

> 1  def x.y.z
> 2  end
```

This is because `exe/syntax_suggest` uses STDOUT.puts while calling `ruby` with the filename uses a fake IO object represented by MiniStringIO. This class was incorrectly not adding a newline to the end of the print.

The fix was to move the class to it's own file where it can be tested and then fix the behavior.

close https://github.com/ruby/syntax_suggest/pull/225

https://github.com/ruby/syntax_suggest/commit/d2ecd94a3b

Co-authored-by: Andy Yong <andyywz@gmail.com>

lib/syntax_suggest/api.rb

@@ -227,6 +227,7 @@ require_relative "lex_all"
 require_relative "code_line"
 require_relative "code_block"
 require_relative "block_expand"
+require_relative "mini_stringio"
 require_relative "priority_queue"
 require_relative "unvisited_lines"
 require_relative "around_block_scan"

lib/syntax_suggest/core_ext.rb

@@ -3,24 +3,6 @@
 # Ruby 3.2+ has a cleaner way to hook into Ruby that doesn't use `require`
 if SyntaxError.method_defined?(:detailed_message)
   module SyntaxSuggest
-    # Mini String IO [Private]
-    #
-    # Acts like a StringIO with reduced API, but without having to require that
-    # class.
-    class MiniStringIO
-      def initialize(isatty: $stderr.isatty)
-        @string = +""
-        @isatty = isatty
-      end
-
-      attr_reader :isatty
-      def puts(value = $/, **)
-        @string << value
-      end
-
-      attr_reader :string
-    end
-
     # SyntaxSuggest.module_for_detailed_message [Private]
     #
     # Used to monkeypatch SyntaxError via Module.prepend

lib/syntax_suggest/mini_stringio.rb

@@ -0,0 +1,24 @@
+module SyntaxSuggest
+  # Mini String IO [Private]
+  #
+  # Acts like a StringIO with reduced API, but without having to require that
+  # class.
+  class MiniStringIO
+    EMPTY_ARG = Object.new
+
+    def initialize(isatty: $stderr.isatty)
+      @string = +""
+      @isatty = isatty
+    end
+
+    attr_reader :isatty
+    def puts(value = EMPTY_ARG, **)
+      if !value.equal?(EMPTY_ARG)
+        @string << value
+      end
+      @string << $/
+    end
+
+    attr_reader :string
+  end
+end

spec/syntax_suggest/unit/mini_stringio_spec.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+require_relative "../spec_helper"
+
+module SyntaxSuggest
+  RSpec.describe "MiniStringIO" do
+    it "#puts with no inputs" do
+      io = MiniStringIO.new
+      io.puts
+      expect(io.string).to eq($/)
+    end
+
+    it "#puts with an input" do
+      io = MiniStringIO.new
+      io.puts "Hello"
+      expect(io.string).to eq(["Hello", $/].join)
+    end
+
+    it "#puts with an input with a newline" do
+      io = MiniStringIO.new
+      io.puts "Hello\n"
+      expect(io.string).to eq(["Hello\n", $/].join)
+    end
+  end
+end