Handle String-like objects for Ripper.sexp

* RSpec relies on this in
  https://github.com/rspec/rspec/blob/rspec-support-v3.13.6/rspec-support/lib/rspec/support/source.rb#L57
  which is given an RSpec::Support::EncodedString.
* CI failure caused by this on truffleruby:
  https://github.com/sporkmonger/addressable/actions/runs/21457707372/job/61802608154#step:7:14

Author: eregon

lib/prism/translation/ripper.rb

@@ -480,7 +480,7 @@ def self.lex_state_name(state)
 
       # Create a new Translation::Ripper object with the given source.
       def initialize(source, filename = "(ripper)", lineno = 1)
-        @source = source
+        @source = source.to_str
         @filename = filename
         @lineno = lineno
         @column = 0

test/prism/ruby/ripper_test.rb

@@ -145,6 +145,14 @@ def test_tokenize
       assert_equal(Ripper.tokenize(source), Translation::Ripper.tokenize(source))
     end
 
+    def test_sexp_coercion
+      string_like = Object.new
+      def string_like.to_str
+        "a"
+      end
+      assert_equal [:program, [[:vcall, [:@ident, "a", [1, 0]]]]], Translation::Ripper.sexp(string_like)
+    end
+
     # Check that the hardcoded values don't change without us noticing.
     def test_internals
       actual = Translation::Ripper.constants.select { |name| name.start_with?("EXPR_") }.sort