Find by message-id should optionally return all matches

Message-ids can sometimes be duplicated for different emails, either within the same list or in different ones.

There should be an option to return all matching messages.

Start by locating the existing find-by-message-id entry point and its tests, since no file paths are named in the issue. Verify how duplicate message-ids are currently handled, then define the option's behavior for matches within one list and across different lists; done means an opt-in query returns all matching messages without changing the default behavior.

Written by the indexing model from the issue text.