Make sure the message format doesn't change

When `use_message_serializer_for_metadata` is false, the message
format should be enveloped in the same way it was in Rails 7.0
to make sure we don't have inconsistent formats.

activesupport/lib/active_support/messages/metadata.rb

@@ -32,7 +32,7 @@ def serialize_with_metadata(data, **metadata)
 
           if has_metadata && !use_message_serializer_for_metadata?
             data_string = serialize_to_json_safe_string(data)
-            envelope = wrap_in_metadata_envelope({ "message" => data_string }, **metadata)
+            envelope = wrap_in_metadata_legacy_envelope({ "message" => data_string }, **metadata)
             serialize_to_json(envelope)
           else
             data = wrap_in_metadata_envelope({ "data" => data }, **metadata) if has_metadata
@@ -68,6 +68,13 @@ def wrap_in_metadata_envelope(hash, expires_at: nil, expires_in: nil, purpose: n
           { "_rails" => hash }
         end
 
+        def wrap_in_metadata_legacy_envelope(hash, expires_at: nil, expires_in: nil, purpose: nil)
+          expiry = pick_expiry(expires_at, expires_in)
+          hash["exp"] = expiry
+          hash["pur"] = purpose
+          { "_rails" => hash }
+        end
+
         def extract_from_metadata_envelope(envelope, purpose: nil)
           hash = envelope["_rails"]
 

activesupport/test/messages/message_verifier_metadata_test.rb

@@ -86,6 +86,20 @@ class MessageVerifierMetadataTest < ActiveSupport::TestCase
     end
   end
 
+  test "messages keep the old format when use_message_serializer_for_metadata is false" do
+    # Message generated by Rails 7.0 using:
+    #
+    #   verifier = ActiveSupport::MessageVerifier.new("secret", serializer: JSON)
+    #   legacy_message = verifier.generate("legacy", purpose: "test")
+    legacy_message = "eyJfcmFpbHMiOnsibWVzc2FnZSI6IklteGxaMkZqZVNJPSIsImV4cCI6bnVsbCwicHVyIjoidGVzdCJ9fQ==--53b1fc02f5b89b2da8c6ce94efa95f5cb656d975"
+
+    verifier = ActiveSupport::MessageVerifier.new("secret", serializer: JSON)
+
+    using_message_serializer_for_metadata(false) do
+      assert_equal legacy_message, verifier.generate("legacy", purpose: "test")
+    end
+  end
+
   private
     def make_codec(**options)
       ActiveSupport::MessageVerifier.new("secret", **options)